To the Editor: Partial properdin deficiency has been described as an innocuous finding.1 In a study of the complement profiles in patients with IgA nephropathy, we have found several subjects with familial partial deficiencies of individual complement component or control proteins,2 including the subject of this report. The proband was a 21-year-old man referred for evaluation of microscopic hematuria, proteinuria, and moderate hypertension. The serum creatinine concentration was 1.4 mg per deciliter, and 24-hour urinary protein excretion was 2.2 g. Biopsy of a kidney revealed findings typical of IgA nephropathy: mesangial deposition of IgA, IgM, IgG, and C3 on direct. © 1981, Massachusetts Medical Society. All rights reserved.