The cathodoluminescence mode of the scanning electron microscope is used for the first time to investigate the lateral dependence of the electron-hole pair generation by the electron beam of the scanning electron microscope m semiconducting material. A multiple-quantum-well structure acts as a detector to measure the relative number of generated carriers by their radiative recombination. The method, which avoids the effect of carrier diffusion, enhances the resolution of the measurement to 50 nm and should prove a help for the quantitative interpretation of cathodoluminescence evaluations.