Sera from 31 healthy donors over age 70 had significantly higher levels of IgM rheumatoid factor, measured by radioimmunoassay, than sera from 27 young controls (25,400 ± 7867 ng/ml vs 2644 ± 347 ng/ml, mean ± SEM, p<0.0001). Pokeweed mitogen-stimulated peripheral blood lymphocytes from healthy donors over age 70 also produced significantly higher amounts of IgM-rheumatoid factor in vitro than lymphocytes from young controls. For individuals over 70, there was a significant correlation between levels of IgM RF in the sera and the levels produced in vitro (r = 0.43, p = 0.02). We investigated the control of IgM RF production by performing co-culture experiments of B cells with either T cells or T cell subsets from subjects over 70 and young controls. T cells from old individuals provided more help than T cells from young individuals to B cells from either young or old individuals. When helper and suppressor T cells subjects were isolated using the monoclonal antibodies OKT4 and OKT8, we demonstrated an increased helper activity of OKT4(+) cells from old subjects, whereas the suppressor activity of OKT8(+) cells did not differ between old and young subjects. In contrast to increased helper T cell activity, it appeared that B cells from old subjects were less capable of rheumatoid factor production than B cells from young controls given the same stimulus. B cells from young individuals produce more IgM-RF than B cells of subjects over 70 in the presence of T cells from subjects over 70 (92 ± 52 vs 31 ± 54, p = 0.04) or in the presence of OKT4(+) T cells from either young or old donors. In addition, in experiments using enriched B cell preparations cultured with helper factor supernatants but without T cells, the B cells from young subjects produced significantly more rheumatoid factor than B cells from old subjects (p < 0.01). Thus, the increased IgM-RF production by old lymphocytes is due to an altered functional state in the old T cell population; the old B cells are actually less capable than young B cells of producing IgM-RF, given the same T cell help or helper factor. These results are consistent with a primary failure of B cell function with age with a resultant homeostatic increase in T helper cell function.