Twenty-one patients with advanced, measurable, renal cell carcinoma (RCC) were administered recombinant interferon alfa-2a (rIFN-alpha 2a) (Roferon-A; Roche Laboratories, Nutley, NJ) intramuscularly beginning at 3 x 10(6) units and escalating to 36 x 10(6) units, 5 d/wk for a total induction period of 14 weeks. rIFN-alpha 2a antibody production was measured using an enzyme immunoassay (EIA). Those sera found to be positive for presence of antibody by the EIA were tested for the presence of neutralizing antibodies (NA) by an antiviral neutralization bioassay (ANB). All patients were evaluable for toxicity, and 19 were evaluable for response and for incidence of antibody formation. Five patients (26%; 95% confidence interval, 6% to 46%) had complete responses (CR) or partial responses (PR) with a median duration of 283 days. An additional ten patients (53%) had minor tumor regressions with a median duration of 86 days. Fifty-one percent of evaluable patients are alive at 18.6 months. Antibodies to rIFN-alpha 2a as measured by the EIA, were detected in 12 (63%) patients. NA were measured in the serum of six (50%) of those EIA-positive patients. Overall, six of 19 patients (32%) developed NA. Median time to the development of antibody as measured by EIA or NA was 8 and 14 weeks, respectively. Median NA titer was 1,200 IFN neutralizing U/mL. NA-positive and -negative patients had a median duration of response of 13.7 v 9.9 months, and survival of greater than 21.3 v 18.3 months, respectively. Clinical toxicity was mild and not therapeutically limiting. Autoantibody production (ANA, rheumatoid factor [RF], Coombs' direct/indirect) occurred in both NA-positive and -negative patients. The clinical significance of the antibodies to rIFN-alpha 2a and the associated autoantibody formation remain unclear; however, presence of antibody was not associated with adverse clinical sequelae.