Sera from 55 patients with positivity for anti-HTLV III antibodies, including 24 "healthy" subjects, 29 LAS and 2 AIDS, were examined by means of high resolution agarose gel electrophoresis and immunofixation, in order to evaluate the occurrence of monoclonal-oligoclonal immunoglobulin patterns. Monoclonal-oligoclonal immunoglobulins were found in 20.8% of "healthy" subjects and in 72.4% of LAS patients. Furthermore, a lower but significant prevalence of monoclonal-oligoclonal immunoglobulins was found in a control group including patients with high-titre positivity for anti-CMV and anti-EBV antibodies. Considering the consistent association between EBV-CMV and HTLV III infection, it is concluded that at least a part of monoclonal-oligoclonal immunoglobulins found in LAS-AIDS patients are likely to be induced by these associated viral infections. On the other hand, the finding of monoclonal-oligoclonal immunoglobulins in "healthy" HTLV III-positive subjects points out a possible direct stimulatory effect of HTLV III on B-cell lineage.