We studied the in vitro effects of the insecticide lindane (2.5-50 microM) on macrophage activating factor (MAF) production by the peripheral blood leukocytes (PBLs) in rainbow trout. The MAF production induced by the mitogens concanavalin A (ConA) and phorbol-12-myristate-13-acetate (PMA) was not modified by lindane pre-treatment. But lindane alone (2.5-25 microM) stimulated the secretion of MAF by PBLs. Intracellular calcium levels ([Ca2+]i) was measured over 6 min by spectrofluorimetry using Indo-1/AM as fluorescent probe. Lindane (25-100 microM) significantly increased the [Ca2+]i in PBLs, but had no effect on calcium at the dose that caused MAF secretion. Moreover, the effect of lindane on MAF production was potentiated by the inhibitor of phosphodiesterase, isobutylmethylxanthin (IBMX). Lindane also directly increased adenosine monophosphate cyclic (cAMP) in PBLs over the same concentration range that it stimulated MAF production by PBLs. Taken together, these results suggest that lindane increase MAF production by acting on intracellular cAMP concentrations. Moreover, the capacity of this insecticide to act on the [Ca2+]i or on the intracellular concentrations of cAMP according to the dose used could possibly explain its contradictory effects earlier observed on immunity.