Recombinant proteins are gaining in importance for therapeutic applications. The proteins are expressed in stable cell lines, within which recombinant DNA has incorporated into the host cells genome. Identification and isolation of extremely high producers from transfected cell populations is a tedious and time-consuming effort. We proposed two different approaches for the establishment of recombinant cell lines: affinity cell sorting and microinjection. Even with the help of single cell sorting and analysis systems (flow cytometry) rarely more than thousand different cell clones, stably expressing a desired gene can be evaluated. We are considering a new approach in which polyclonal cell populations will be established which co-express a protein of interest and an epitope-tagged cell surface receptor. A microbead fixed antibody specific to the epitope tag is hoped to assist in the isolation of rare, but highly productive cells from large suspensions of transfected cells. For this reason we have also developed an efficient method for the bulk transfection of CHO cells. On the side of vector-development for this approach, we have chosen to investigate the utility of a mouse serotonin 5-HT3 receptor as a cell surface marker. This receptor can be expressed at very high levels (up to 1 x 107 receptors per cell) in heterologous systems. The receptor is detectable on the cell surface by means of fluorescent ligands, which allows the determination of transfection efficiencies in suspension cultures. A Flag-tag was fused to the amino terminal end of the receptor without adverse effects on receptor function or ligand binding. Only 5 % of receptor DNA in the transfection cocktail was sufficient to detect positive cells in a population. The specific interaction of the epitope-tagged receptor with the cognate antibody fixed to microbeads was tested in comparison to non-transfected control cells using laser scanning confocal microscopy. A 80% enrichment of cells expressing GFP and the 5-HT3 receptor was achieved in 30 minutes. We have investigated microinjection as a gene transfer method for the establishment of recombinant mammalian cell lines. The conditions for the injection of plasmid DNA into either the nucleus or the cytoplasm of CHO-DG44 cells were established using pMYK-EGFP, an expression vector with the enhanced green fluorescent protein (EGFP) gene under the control of the murine CMV promoter and the puromycin resistance gene for selection. To estimate plasmid copy number during microinjection, it was first necessary to determine the injection volume by fluorescence quantification of injected FITC-dextran. For microinjection into the cytoplasm the volume ranged from 200 to 350 fl and for nuclear microinjection it was about 180 fl. We then investigated the impact of DNA concentration and injection pressure on transient EGFP transient expression following microinjection into the cytoplasm or nucleus. As a general rule, we observed that concentration of the DNA solution injected into the cells was the most important parameter regardless of the pressure or the volume of injection. Recombinant cell lines expressing intracellular EGFP were established following microinjection into the nucleus and the cytoplasm with linear and circular DNA. Selection with puromycin, clonal expansion, and cell banking were completed within three to four weeks post-microinjection. In parallel, recombinant cell lines were established after calcium-phosphate transfection with linear and circular DNA. The number of plasmid copies integrated was determined by Southern blot and the number and localization of integration sites was determined by fluorescence in-situ hybridization (FISH). Growth of the cells and stability of EGFP expression were investigated by cultivation over a two-month period. This approach presents several advantages over other DNA delivery methods such as control of the quantity and localization of DNA delivered into the cell and speed of recovery of recombinant cell lines. This approach may be relevant for the establishment of recombinant cell lines from cells that are not easily transfected by classical methods.