High concentrations of the non-steroid anti-inflammatory drugs, alone and in association, reduce the blastogenesis of human lymphocytes in vitro. This effect is probably due to the toxicity of these agents and not to the inhibition of prostaglandin (PG) prosuction. Therefore drugs, such as, salicylates and associations of salicylates with other non-steroid anti-inflammatory drugs, which have a weak action on PG-synthesis, also inhibit proliferation of lymphocytes.