The effect of four polyene antibiotics, Candicidin, Etruscomycin, Filipin and Pimaricin upon mouse lymphocytes was studied. Polyene antibiotics are known to form aqueous pores in the cell membranes inducing known anion or cation selective fluxes. Candicidin was capable of inducing marked DNA-synthesis and polyclonal antibody production when added to normal spleen cells. Etruscomycin and Pimaricin showed a weak inconsistent DNA synthetic stimulatory effect, whereas Filipin was found to be totally uneffective. The stimulating property of Candicidin was also demonstrated on spleen cells from nude mice whereas there was no effect on cortisone resistant thymocytes or spleen cells passed through a nylon fibre column. Thus we conclude that Candicidin is a PBA for mouse lymphocytes. We have previously reported that the two anionselective polyenes, Nystatin and Amphotericin B, are polyclonal B-cell activators for mouse lymphocytes and in this paper the possible mechanism of triggering is further discussed.