Bacteria-antibody complexes were used to study capping of receptors for the activated third component of complement on peripheral blood lymphocytes. The impairment of C3d receptor capping improves in patients with chronic lymphocytic leukemia responding to therapy, but this does not necessarily correspond to the fall in peripheral lymphocytosis. Following lymphapheresis, increased capping appears to be due to selective removal of noncapping cells, but the mechanisms of improvement remain unexplained following other therapeutic measures. Lymphoma capping at 37 degrees C was normal in untreated patients but impaired after treatment, suggesting that this is related to the therapy itself. A high proportion of lymphocytes had receptors in the capped configuration in the basal state before incubation. In non-Hodgkin's lymphoma this was unrelated to treatment status, but it occurred only in untreated patients with Hodgkin's lymphoma with no further increment in capping after incubation. These receptors may exist in the aggregated state without the addition of exogenous ligand.