The production of anti-tetanus toxoid antibody and rheumatoid factor (RF) by individual peripheral blood mononuclear cells (PBMC) was studied at various times after booster immunization of healthy volunteers with tetanus toxoid (TT). TT-specific antibody-secreting cells and RF-secreting cells were assayed by the enzyme-linked immunospot (ELISPOT) assay. Cells obtained 5-9 days after immunization were found to spontaneously secrete TT-specific antibodies. This response was specific for the immunogen, was abrogated by cycloheximide and accounted for most of the immunoglobulins produced. TT-specific antibody-secreting cells were found concomitantly with cells secreting RF, mainly of the IgM isotype. Elevated levels of circulating immune complexes (CIC) containing IgG and C3 were observed shortly before the appearance of both TT-specific antibody-secreting cells and IgM RF-secreting cells. Thus, after booster immunization with TT, two major subpopulations of immunoglobulin-secreting cells appear concomitantly in the peripheral circulation: a population of cells secreting TT-specific antibody and a population of cells secreting IgM RF. It is suggested that RF production in man represents a physiological event which might be triggered by IgG-containing immune complexes formed during a secondary immune response.