Nephritic factor (NF) was isolated from serum T.A., NF(TA), as we have previously described, including selective binding to and decay-release. After two such cycles, 90% of the protein bound to compared to 2% specific binding of partially purified material consisting only of IgG. Purified NF(TA) did not bind to EC3b in absence of B and D, nor to C3bINA and β1H-treated EC3b in presence of B and D. The chain structure of NF was examined by SDS PAGE of the reduced protein after iodination using the chloramine-T (CT) or the Bolton Hunter (BH) method. According to the distribution of radioactivity, the subunit size of CT labeled NF was 60,000 daltons (92%) and the subunits of BH labeled NF were 60,000 (67%) and 25,000 daltons (33%). IgG iodinated by either method showed subunits of 52,000 (67%) and 25,000 daltons (33%).