Levels of complement components and the presence of immune complexes were determined in blood samples from 23 patients is a function of time after kidney transplantation. During the first three post-transplantation weeks a decrease in the concentration of plasma C3 with a simultaneous increase of one of its breakdown products (C3d) was generally observed. This pattern often accompanied acute rejection episodes beyond 4 weeks after transplantation, while in the absence of complications normal and stable levels prevailed. In contrast, the presence of circulating immune complexes appeared not to correlate with rejection reactions. All 7 cases with detectable immune complexes presented with various concomitant neoplastic (renal carcinoma, Kaposi sarcoma) or infectious diseases (pneumonia, septicaemia, Herpes zoster or Cytomegalovirus infection). Thus, monitoring of plasma C3 and C3d may represent a helpful additional criterion for the assessment of acute rejection in recipients of kidney allografts; the presence of circulating immune complexes, although not correlating with graft rejection, may be taken as a sign of complicating additional disease.