Abstract In acromegalic patients we have previously described a low ability of B-lymphocytes to differentiate into plasma cells under PWM stimulation, and a decreased chemotaxis of polymorphonuclear leukocytes (PMN) towards N-formylmethionylphenilalanine (FMP). In this study we examined the effect of exogenous GH over these immune functions in normal cells. PMN were purified by dextran sedimentation, incubated with recombinant human GH (0 to 20 ng/ml) and subjected to stimulation with FMP. PBMC were cultured with or without PWM, in the presence of GH (between 0 and 100 ng/ml). Plasma cells were determined as hemolysis forming cells and also by immunofluorescence. GH, in a dose-dependent way, decreased directed migration of PMN (5 ng/ml: 1.787 ± 148 μm; 10 ng/ml: 1.581 ± 221 μm; 20 ng/ml: 1.569 ± 149 μm, all as mean ± S.E.M.), when compared to similar values of untreated PMN (0 ng/ml 2.085 ± 139 μm). GH treatment did not modify spontaneous migration. Net migration showed the same pattern as directed migration. GH decreased dose-dependently the PWM-driven differentiation of B-lymphocytes into plasma cells to 60% of the basal level. Although not significantly, GH tended to increase spontaneous B-cell differentiation. These results could account for the already described defect in B-cell differentiation and PWN chemotaxis in acromegaly, emphasizing the relationship between the endocrine and immune systems.