A sensitive in vitro bioassay for the alpha-interferon induction of lupus-type inclusions (LI) has been established with the human B lymphoblastoid cell line, Daudi. Sera from 11 patients with systemic lupus erythematosus (SLE) were evaluated with this assay. LI induction by these sera increased in proportion to their antiviral activity on Madin-Darby bovine kidney (MDBK) cells. Two of these sera did not induce LI; they showed no antiviral activity on the MDBK cell assay. Clinically and serologically, their donors were in remission. Two sera induced the formation of LI that exceeded the maximum frequencies obtained with 3 alpha-interferon preparations. These sera had the greatest antiviral activities, and their donors had the greatest disease activities. Antisera to alpha-interferons prevented the induction of LI with the pure and homogeneous recombinant human leukocyte interferon, IFLrA, and SLE sera. Together, these results provide evidence that the alpha-interferon endogenous to SLE patients has a great ability to induce LI, and the SLE serum induction of LI corresponds well to the patient's disease activity.