Sera from 168 patients with various types of chronic glomerulonephritis (GN) were assayed for immune complexes (IC) by two independent methods, a modification of the PEG precipitation test (PEG) and the inhibition of complement-dependent lymphocyte rosette formation (RI). Both assays had different reactivities, the RI being more sensitive than the PEG test. Higher percentages of positivity of these tests were observed in the GN groups than in normal controls. However, serial measurements demonstrated that IC were present only intermittently in most instances. The presence of IC correlated with disease activity in patients with focal glomerulosclerosis membranous GN and membranoproliferative GN, while in lupus erythematosus it reflected the effects of different treatment regimens although it had no relationship with clinical symptoms. Nevertheless, our results suggest that these tests are of little help to the clinician to appreciate disease activity and monitor therapy in individual GN patients.