In order to investigate the effects of growth hormone on human B cells, we studied immunoglobulin synthesis and [3H]-thymidine uptake by B cell lines, IM-9, GM-1056 and CBL, and purified tonsil B cells. Cells were cultured in a serum- and albumin-free medium, Cosmedium-001 to exclude interaction between a certain growth-promoting substance and any unknown substances that serum may contain. GH enhanced IgG synthesis and [3H]-thymidine uptake by IM-9 cells, IgA synthesis by GM-1056 cells and IgM synthesis by CBL cells in a dose-dependent fashion. This was not a non-specific effect of protein in GH, since equivalent or higher concentrations of proteins such as bovine serum albumin, interleukin-2, interleukin-5, granulocyte/macrophage colony-stimulating factor, or erythropoietin did not stimulate B cell lines. Moreover, the rabbit anti-GH antibody blocked the enhancing effect of GH, while normal rabbit antibody failed to do so. GH also enhanced immunoglobulin synthesis (IgG and IgM) and thymidine uptake by Staphylococcus aureus Cowan strain I-activated tonsil B cells. These results indicate that GH has a direct stimulating effect on B cells, and, in addition to its endocrinological function, GH may also act as an immunoregulatory cytokine.