We investigated the influence of human growth hormone (hGH) on mitogen-stimulated lymphoproliferation, in vitro IgM production, serum levels of immunoglobulins, somatomedin-C (Sm-C) values and serum growth-promoting activity (Thymidine Activity, TA) in 18 short children, aged between 6.6-14.5 years, undergoing a 3-month course of hGH therapy. Blood was collected the day before treatment (Group A), on the 5th day after patients were administered hGH daily (0.1 U/kg) i.m. for 4 days (Group B), after a 3-month course of hGH injected three times weekly, and finally before (Group C) and 24 h after an extra injection (Group D). In vitro IgM production from the patients' unstimulated lymphocytes decreased from 277 +/- 41 (Group A) to 168 +/- 38 (Group B), to 119 +/- 43 (Group C) and then to 119 +/- 28 ng/ml (Group D) (p less than 0.05). Using PWM-stimulated lymphocytes in vitro IgM production decreased from 2,015 +/- 464 (Group A) to 1,116 +/- 316 (Group B), then to 511 +/- 170 (Group C) and 968 +/- 295 ng/ml (Group D) (p less than 0.02). The variation of this decrease could be correlated with the variation of growth velocity during treatment (r = 0.619, p less than 0.05). In contrast, no significant changes were found following therapy either in serum levels of IgA, IgE, IgG, IgM, Sm-C and TA, or in phytohemagglutinin, concanavalin A and pokeweed mitogen-stimulated lymphoproliferation. Our data suggest that there is some relationship between growth hormone, growth and immunity.