The complement functions of 42 patients with non-Hodgkin's lymphoma have been examined. The patients were divided into groups according to the severity of their disease: 1st--patients with high-grade lymphomas, 2nd--with low-grade lymphomas and 3rd--with chronic lymphocytic leukaemia. The adopted methods were the measurements of complement-mediated immune complex solubilizing capacity (CMSC) and the complement-mediated immune complex precipitation inhibition capacity (IPIC). The CMSC and IPIC values were examined parallel with CH50, C3 complement levels and with levels of circulating immune complexes (CIC) in the sera of patients. The results indicated that the acquired deficiency of complement functions could be established by CMSC and IPIC measurements in the sera of patients with high-grade lymphomas. These defects were found to be milder in the group with low-grade lymphomas, and were not detectable in CLL. The changes of CH50 levels were found to be similar to that of IPIC values and the decrease in C3 levels was detectable in high-grade and low-grade lymphomas too. Elevated CIC levels were found in those cases in which both CMSC and IPIC were decreased.