The effect of the polar solvent dimethylsulfoxide on the synthe-sis and secretion of albumin and alpha-fetoprotein was studied in an establishe line of mouse hepatoma cells (BW1). Incubation of BW1 cells with dimethyl-sulfoxide led to a marked enhancement in the cellular content of albumin and alpha-fetoprotein (42 and 94 % for albumin and 118 and 154 % for alpha-fetoprotein in 0.5 and 1 % dimethylsulfoxide-containing growth medium, respectively) as well as in the extra-cellular accumulation of these proteins.The albumin contribution to total BW1 cell-associated protein increased with exposure to increasing concentrations of dimethylsulfoxide in the medium.In contrast, while the alpha-fetoprotein content, per 106 hepatocytes increased as a function of dimethylsulfoxide concentration, synthesis of alpha-fetopro-tein relative to total cellular protein was maximal in 0.5 % dimethylsulfoxide and was not further stimulated upon exposure of the cells to higher concentra-tions of the polar solvent.