Aim of this investigation was to compare the effects of 10(-4) M and 10(-7) M As compounds on spontaneous and PHA stimulated PBMC proliferation and IFN-gamma and TNF-alpha release. The inhibitory effect of the 10(-4) M As salts was in the following order: momo-methyl-arsinous acic (MMAs(III)) > sodium arsenite (As(III)) > tetraphenyl arsonium chloride (As(V)) > sodium arsenate (As(V)) > potassium- and sodium-esa-fluorum arsenate (As(V)) > dimethyl arsinic acid (DMAs(V)), while monomethyl-arsonic-acid (MMAs(V)) and arsenobetaine did not exert immune effects. 10(-7) M MMAs(III) stimulated the spontaneous PBMC proliferation, while As(III) and DMAs(V) enhanced the PHA stimulated PBMC proliferation. This study shows that the immune effects of As salts depends on speciation; moreover, the immunotoxicity of inorganic arsenic in part depends on the intracellular bio-synthesis of MMAs(III) from MMAs(V).