Circulating immune complexes have been investigated in jirds (Meriones unguiculatus) infected with the filarial nematode Brugia pahangi. Two-month-old male jirds were inoculated with seventy-five B. pahangi infective larvae into the left groin. At 8 months post-infection, sera of individual animals from a group of seventeen infecteds and seventeen age-matched controls were analysed for immune complexes by (1) a solid-phase C1q binding assay (Clq-SP) and (2) precipitation with 3.5% polyethylene glycol followed by binding of 125I-labelled rabbit anti-jird Ig antiserum (PEG). A significant increase in the level of circulating immune complexes was shown in the infected group as compared with the controls for both assays, with a P value = 0.005 for PEG and P = 0.001 for Clq-SP. Using the mean of the control group +/- 2 s.d. as the upper limit of the normal range, 24% of the infected group had elevated immune complex levels by the PEG assay, and 41% were elevated in the C1q-SP assay. A high degree of variability was noted in the levels of immune complexes among individual animals in the infected group by each test. No correlation between immune complex levels and numbers of circulating microfilariae was found in either assay.