The authors report correlative studies of factor B, properdin, C3, and the serum opsonic activity for Escherichia coli and Staphylococcus aureus in patients with the idiopathic nephrotic syndrome (INS). Thirty-two patients with the idiopathic nephrotic syndrome were studied. Twenty-two patients were steroid responsive (Group I), of which 11 patients were steroid dependent. Ten patients were steroid resistant (Group II). The presence of the nephrotic syndrome (regardless of steroid responsiveness) was associated with a significantly reduced mean serum factor B concentration. Of the complement components studied, only factor B was significantly decreased during relapse (factor B 24.8 ± 9 μg N/ml in INS versus 46 ± 12 μg/ml in normal, P < 0.001). When the combined groups were studied during remission of INS, the mean serum factor B concentrations were not different from normal. Similar results were found when each group was examined separately. Ten patients had serum factor B determinations during both exacerbation and remission of the nephrotic syndrome. The serum factor B increased in nine patients and was unchanged in one. A highly positive correlation between serum factor B and serum albumin concentrations was present (r = +0.805, P <0.001). Twenty sera from 14 patients with the nephrotic syndrome, studied at various stages of the nephrotic syndrome, were evaluated for opsonization titers of E. coli employing the bacterial killing assay. Eleven sera from seven patients had reduced capacity for E. coli opsonization, i.e., bacteria were not opsonized in 2% serum. There was a significant difference between the mean factor B concentrations of sera with abnormal as compared to normal opsonization of E. coli (P <0.001). The serum of one patient with nephrosis opsonized E. coli normally 60 min after the addition of isolated factor B. The opsonic activity of serums from three additional patients with relapse of INS were studied by determining uptake of radiolabeled E. coli by normal leukocytes. The addition of isolated factor B increased phagocytosis in the serum from two of three patients. Decreased opsonization of S. aureus was found in only 3 (two patients) of 19 sera. The complement components, C19, C4, properdin, and C3, were normal during both periods of exacerbation and remission with the exception that C4 was significantly elevated during remission of nephrosis. These results suggest that a significant decrease of serum factor B concentration is associated with abnormal opsonization of E. coli. The authors do not know the cause of decreased serum factor B, but renal pathologic findings do not appear to be of etiologic importance. The high correlation of serum factor B with serum albumin concentration suggests that factor B, with a molecular weight of 80,000, was being lost in the urine. The increased incidence of severe infections such as peritonitis, which occurs in INS patients, may be the result of reduced serum factor B, a component of the alternative pathway. Although further work is necessary to determine the cause of factor B decrease in INS and the role of other serum factors in this opsonic defect, the understanding of the role of factor B in the predilection of nephrotic patients to certain infections could be of great importance in the care of these patients.