SUMMARY Eleven patients with renal homografts were studied for changes in immunoglobulin levels and complement activity associated with jection.Eleven episodes of clinical rejection were detected in 8 of the allograft recipients. Eight episodes began within the first postoperative week and 3 were identified more than 3 weeks following transplantation. Each rejection was associated with a significant fall in IgM and C'HM. Equally significant falls in IgG occurred in 5 of the 8 early rejections and 1 of the 3 delayed rejections. Two of 3 patients without overt rejection also demonstrated simultaneous falls in serum unoglobulin and complement levels. Transfusions and urinary immunoglobulin losses were excluded as causes of the immunoglobulin hanges. Immunosuppression and hemodilution may have contributed to some but not all of the immunoglobulin changes. The administration of antilymphocyte globulin, although followed by the appearance of precipitating antibodies in the serum, could not be related to the immunoglobulin and complement depression. Although humoral rejection mechanisms may have caused the closely related falls in IgM and CH50, the large decline of serum IgG levels appears to have been enhanced by nonimmunological factors. The consistent decline in immunoglobulin and complement levels during early and delayed rejection merits further investigation to determine the possible relationship of these declines to humoral mechanisms