The in vivo effect of a calf thymus extract, thymostimulin, on the levels of circulating immune complexes (CIC) and serum lysozyme was evaluated in 32 patients with untreated Hodgkin's disease. Using the platelet aggregation test for detecting CICs, 12 patients (37%) had positive titers before thymostimulin treatment; 3 patients (10%) remained positive following therapy. Serum levels of Clq-binding immune complexes were evaluated (greater than 24.5 micrograms/ml) in 8 patients prior to thymostimulin therapy (mean value: 42.3 micrograms/ml); 3 patients continued to have elevated levels after treatment. Serum lysozyme levels for Hodgkin's patients was similar to control values (10.6 vs. 8.3 micrograms/ml); however, the Hodgkin's patients with initially elevated CICs had a lower serum lysozyme level than patients with initially normal CICs (12.9 vs. 7.3, p less than 0.02). Thymostimulin increased serum lysozyme levels in the Hodgkin's patients in whom the CICs were initially elevated (7.3 vs. 10.4 micrograms/ml, p less than 0.05). These data suggest that thymostimulin exerts an effect on the nonspecific immune system of Hodgkin's disease patients.