We demonstrate that LTA4 (10–100 ng/ml) and LTB4 (10–100 ng/ml) influence the B cell differentiation and isotype switch to a different degree. In contrast to LTB4, LTA4 selectively enhanced the IgG synthesis of B cells and PBMC in the range between 30 and 110% (n = 6, p <0.005). Under the same conditions, both LTs enhance the IL-4-induced CD23 expression of purified B cells by 20–40%. Glucocorticoid (10––7M prednisolone)-treated PBMC, which showed an enhanced Ig(G, A, M, E) synthesis, alter their isotype secretion pattern after LT costimulation. LT costimulation of glucocorticoid-treated cells leads to a suppressed IgG synthesis from 20 to 60% (n = 6, p <0.005). Both LTA4 and LTB4 enhance the IL-4- and glucocorticoid-induced IgE synthesis. Our data thus suggest that LTA4 and LTB4 modulate the immunoglobulin synthesis of PBMC. Both lipid mediators exert different activities on isotype selection.