Normal mouse serum contains H-2 antigen-like components. One such protein consists of a 40000 dalton chain and ~2-microglobulin (P2m) and the other component contains the two H-2 antigen-like subunits and, in addition, two larger polypeptide chains with apparent molecular weights (mol. wt.) of 65 000 and 80 000. Radioimmunoassay procedures specific for the H-2 antigen-like chain and P2m, respectively, have been developed. Whereas free P2m and P2m associated with other polypeptide chains reacted identically in the P2m radioimmunoassay, the two serum forms of the H-2 antigen-like chain displayed different reactivities in the heavy-chain assay. The low mol. wt. protein complex, comprising the 40000 dalton chain and P2m, inhibited in the heavy-chain assay identically with the standard. However, the high mol. wt. complex containing the H-2 antigen-like chain gave rise to a biphasic inhibition profile. This was apparently due to some antigenic determinants of the 40000 dalton chain being buried by the 65 000 and 80 000 dalton chains. Inhibition curves parallel with the standard were, however, obtained on dissociation of the high mol. wt. protein complex with sodium dodecyl sulfate. Thus, by subjecting serum to the ionic detergent-sensitive radioimmunoassay, procedures for the two H-2 antigen-like chains could be developed. The serum levels of the H-2 antigen-like chain and of P2m vary with the age of the animal, and adult levels are recorded only at 6 to 7 weeks of age. The serum concentrations of the two proteins are strongly correlated. The H-2 haplotype appears to partly regulate the serum levels of the two proteins. Mice of the H-2f haplotype contain about 40-fold lower levels of the H-2 antigen-like chain than do mice of other strains.