A new component of the complement (C).system, with a specific binding affinity for the activated Ss-protein (C4) has been identified in mouse serum. This protein, named Ss- (or C4)-binding protein (Ss-bp), was purified about 200 times from mouse plasma. Ss-bp is a heat stable (56 degrees C, 60 rain) beta-globulin with a sedimentation coefficient in sucrose density ultracentrifugation of 10s. Its concentration in serum of adult male and female mice is 160 and 60 mug/ml, respectively. In EDTA-plasma, Ss and Ss-bp are not associated and can be separated by chromatography in Sephadex G-200. However, in serum Ss-bp binds tightly to Ss. The bonds between these proteins cannot be reversed by chelation of divalent cations. As a consequence of the formation of Ss/Ss-bp complexes, the properties of Ss-bp appear to be quite different in serum of mice with high (Ss-H) or low (Ss-L) levels of Ss-protein. In Ss-H serum, all of Ss- bp is bound to Ss. In Ss-L serum, Ss-bp is mostly free. Because the electrophoretic mobilities of free and complexed Ss-bp are quite different, Ss-bp appears to be polymorphic in serum (but not in EDTA- plasma). The strict dependency of the apparent electrophoretic mobility of Ss-bp on the levels of Ss in serum was demonstrated in a series of congenic mice and among the progeny of a cross between Ss-H and Ss-L strains of mice. Without exception, the slow and fast varieties of Ss-bp were associated with the Ss-L and Ss-H traits. Ss-bp of the slow variety can be transformed into the fast variety by addition of pure human C4, or C4-sufficient guinea pig serum, to Ss-L serum. In both instances Ss-bp formed stable complexes with C4 or a C4- derived peptide. These findings highlight the binding specificity of Ss- bp for the fourth component of the complement system, and in addition they demonstrate a functional homology between the Ss-protein and C4 from two different species.