Antipsychotic drugs are widely used to alleviate a number of psychic disorders and have been found to modulate some immune parameters, but the molecular mechanism of their action on the proliferative activity has been poorly recognized. In the present study, we investigated effects of various antipsychotics on the proliferative activity of lymphocytes stimulated by concanavalin A (Con A) and lipopolysaccharide (LPS). Chlorpromazine (3 x 10(-6)-10(-4) M) showed the most potent effect in inhibiting 3H-thymidine incorporation into C57BL/6 mouse spleen cells stimulated by Con A and LPS. Treatment of the cells with thioridazine (10(-5)-10(-4) M), promazine (10(-5)-10(-4) M), haloperidol (10(-5)-10(-4) M), risperidone (10(-5)-10(-4) M), raclopride (3 x 10(-5) - 10(-4) M), remoxipride (3 x 10(-5)-10(-4) M) and clozapine ( 3 x 10(-5)-10(-4) M), but not with sulpiride (10(-7)-10(-4) M), suppressed proliferative activity of splenocytes after Con A stimulation. On the other hand, LPS-induced proliferation of splenocytes was inhibited by clozapine, promazine, thioridazine and haloperidol, but not by risperidone, remoxipride, sulpiride and raclopride. In the next part of the study, the influence of some kinase modulators on chlorpromazine- and clozapine-evoked inhibition of the proliferative activity of splenocytes was determined. Wortmannin, a selective phosphatidylinositol 3-kinase (PI3-K) inhibitor, blocked chlorpromazine and clozapine inhibitory effect on the mitogen-stimulated splenocyte proliferation. The involvement of PI 3-K /protein kinase B (PKB, Akt) pathway was confirmed by the results of the Western blot study, which showed that both drugs increased the level of active phospho-Ser-473 Akt, without changing the total Akt level, and decreased the level of active, nonphosphorylated glycogen synthase kinase-3 (GSK-3beta). Additionally, we have found that chlorpromazine action was also attenuated by a selective p-38-MAPK inhibitor, while clozapine effect was suppressed by a protein kinase C (PKC) activator. The obtained results indicated that atypical antipsychotic drugs markedly inhibited the proliferative activity of splenocytes only after ConA stimulation. Inhibition of the proliferative capability of splenocytes by chlorpromazine and clozapine resulted mainly from the activation of PI3-K/Akt pathway.