The binding protein of the fourth component of complement (C4-BP) is a regulatory protein of the complement system with specific affinity for the fourth component. This paper describes a structural polymorphism of murine C4-BP and its linkage to the major histocompatibility complex of the mouse (H-2). After isoelectric focusing of whole mouse plasma in low-endosmosis agarose, C4-BP was demonstrated as a single precipitin band by overlaying monospecific antiserum on the agarose gel. Two C4-BP patterns were distinguished among many strains of mice on the basis of isoelectric point--C4-BP a type, which has a pH range of 6.5-7.0 (exemplified by B10.BR and B10.AKM), and C4-BP b type, which has a pH range of 6.3-6.6 (exemplified by B10 and B10.M). Genetic crosses between two strains bearing distinct C4-BP types demonstrate a C4-BP pattern representative of both types. A linkage study was carried out in which progeny of two backcross combinations--[(B10 X B10.BR)F1 X B10.BR] and [(B10.AKM X B10)F1 X B10.AKM]--were phenotyped for C4-BP type and serum fourth-component level. Results were obtained suggesting that C4-BP patterns are inherited by a single codominant locus (C4-Bp) linked to the H-2 complex. The recombination frequency between the C4-Bp locus and the S region was 0.017. By phenotyping appropriate intra-H-2 recombinants of three different backgrounds (B10, A, and HT), this locus was assigned to the right of the H-2D region.