Polyclonal immunoglobulins (Ig) were measured at diagnosis and/or following chemotherapy in 226 patients with a malignant plasma cell dyscrasia (PCD), including 11 patients with solitary myeloma (SM) and 215 patients with multiple myeloma (MM). At diagnosis, Ig synthesis suppression was observed in 80.7% of patients with MM but never in case of SM (p < 0.001). In patients with MM, there was a clear correlation between IgA or IgM levels (but not IgG) and the total body burden of myeloma cells (p < 0.01), the lowest levels being observed in patients presenting with the highest myeloma cell mass. Of major interest, for patients evaluated following the induction of chemotherapy, an increase of Ig, from low to normal levels, was only noted in case with a myeloma cell mass regression over 90% and successful achievement of a > 1-year plateau period. We concluded that polyclonal Ig evaluation appeared to be of diagnostic and prognostic values in the management of malignant PCD.