We studied the production of chemiluminescence (CL) by peripheral blood neutrophils from 24 normal subjects, 13 lung cancer patients with clinical stages (CS) III and IV, and 27 breast cancer patients with CS II, III, and IV. Evaluations were made before chemo- and radiotherapy treatments. CL was expressed as: baseline of the record background activity; area under the curve (AUC) of opsonized zymosan response; maximum peak (MP) response; time to MP (TMP); and total time (TT). Simultaneously, circulating immune complexes (CIC) in sera were evaluated by polyethylene glycol precipitation technique. The results demonstrated that lung cancer patients with CS III had an increase of TMP and TT values, while breast cancer patients with CS IV showed the lowest values of AUC and MP compared with the control group. The CIC levels were increased in all the cancer patients. There was no correlation between the baseline CL activity and the levels of immune complexes.