Serum and urine samples from seven recombinant inbred mouse strains, derived from a cross between BALB/c and C57BL/6, were examined to determine the immunoglobulin heavy chain (IgCH) and the major urinary protein (MUP) allotypes. CXBG and CXBJ exhibited the same IgCH alleles as did BALB/c; the others resembled C57BL/6, thus providing no evidence of crossover types. Comparison of the Mup and brown coat colour (b) alleles (both on linkage group VIII) revealed that three of the strains resemble BALB/c and two resemble C57BL/6, whereas the CXBE and CXBI strains are crossover types.