A comparative study of the serum levels of an immune-adherence inhibiting factor was carried out on serum samples from eighty-seven nephrotic syndrome children, twenty-eight nephrotic adults, 202 normal school children, 116 blood donors, twenty-five falciparum malaria children and 172 patients with miscellaneous diseases. Low titres (1/32 and below) of the factor were present in sera from 63.2% of the nephrotic children 60.7% of nephrotic adults and 60.0% of children malaria, as compared with 30.7% of the normal children, 25.5% of the patients with miscellaneous diseases and 41.4% of the blood donors. There is a significant difference between nephrotic children and normal children with low titres (P less than 0.05). Furthermore, 36.8% of the nephrotic children had serum titres of 1/4 or less, as compared with 6.4% of normal children. The serum factor is tentatively referred to as 'C3b-inase'. Its similarity to conglutinogen-activating factor (KAF) and its possible role in the pathogenesis of the immune-complex nephropathy of childhood nephrotic syndrome associated with malaria are discussed.