Novel pyrrolidine 3,4-diol derivatives of the type (2R and 2S,3R,45){[(( 1R)-1-methoxycarbonyl and hydroxylmethyl)-1-arylmethyl)amino] ethyl}-pyrrolidine-3,4-diol have been prepared and evaluated as a-mannosidase inhibitors and assayed for their anticancer activity in vitro. They all exhibit specific but moderate activity as inhibitors towards a-mannosidase from Jack beans. Compounds 7 and 8b bearing hydroxymethyl and trifluoromethylbiphenyl groups show the best antiproliferative effect in two pancreatic cancer cell lines.