Peripheral blood mononuclear cells from three patients with multiple myelomas (M.M.), four patients with Essential Mixed Cryoglobulinaemia (E.M.C.) and four normal adults as controls were cultivated with or without poke weed mitogen (PWM); in vitro immunoglobulin (Ig) production was measured after seven days using the enzyme-linked immunosorbent assay method (ELISA). No significant differences in Ig production were noted between patients and controls: E.M.C. patients, on the other hand, displayed considerable spontaneous IgM production (without appreciable differences, however, between spontaneous and PWM-stimulated production). M.M. patients, on the other hand, displayed only scanty spontaneous IgM production and in two cases the PWM response was completely lacking. These results were then integrated and correlated with a parallel study of similar groups of patients and controls carried out with the aim of investigating modifications in the distribution of B and T lymphocyte subpopulations. Possible interpretations of the results are discussed.