Synovial fluid (SF) from rheumatoid arthritis patients (RA) and culture-supernatants of synovial tissue (ST) cells from RA patients and juvenile rheumatoid arthritis (JRA) patients were examined for their ability to promote B cell growth. SF was not suitable for studying B cell growth promotion because with the anti-mu driven assay system employed, all 15 samples strongly inhibited B cell proliferation. Supernatants of in vitro unstimulated ST cells from RA and JRA patients affected B cell growth in different ways, ranging from strong inhibition to moderate stimulation. Supernatants of unstimulated peripheral blood mononuclear cells (MNC) from healthy donors did not influence B cell proliferation. After phytohaemagglutinin (PHA) stimulation of the ST cells and normal MNC, culture supernatants of RA ST cells and normal MNC all stimulated B-cell growth, while culture supernatants of PHA-stimulated JRA ST cells displayed a variable picture. The differences between PHA-supernatants from RA, JRA and normal MNC were not statistically significant. These results indicate that the inflamed synovia of JRA and RA patients contain cells that can produce soluble factors with B cell growth promoting activity. Some of the data in the study suggest that these factors are produced in vivo and thus might be responsible for the observed B-cell activation in joints of these patients.