An apparently completely complement C7-deficient patient with refractory otitis media and two episodes of meningococcal disease was given therapeutic plasma transfusions in 1992 and 1994. Following these transfusions unexpected changes were found in C7 levels. Immediately after transfusion the serum C7 levels failed to rise to the expected levels but then rose to 5-10% of the normal mean during the next 5 days and remained at that level for more than 2 weeks before eventually returning to zero. The patient's DNA genotyped C7 M, and therefore C7 N donor plasma was selected for the second transfusion to allow identification of the source of the C7 circulating post-transfusion. This C7 phenotyped C7 M, demonstrating it to be of recipient origin. Therefore, the apparently completely C7-deficient patient was able to secrete some C7. By a combination of DNA typing and isoelectric focusing of the C7 appearing after transfusion, it was demonstrated that the patient was heterozygous for combined subtotal C6/C7 deficiency (inherited from his father) and a different, so far uncharacterized, subtotal C7 deficiency (inherited from his mother). The low amount of C7 secreted appeared to be constantly consumed, probably by generation of C5b6 as a result of his chronic infection. He had been shown to have circulating C5b6 most of the time, and thus only when sufficient exogenous C7 was given to consume the free C5b6 did his own C7 appear in circulation.