Polymorphism of tear proteins was found by agarose gel electrophoresis among inbred strains of rats. The proteins (RTP-1) are inherited as a single autosomoal trait. The locus was designated Rtp-1 (rat tear protein-1) and it had two codominant alleles (Rtp-1a, Rtp-1b). Although we did not find any recombinant between the Rtp-1 and the Mup-1 loci among 67 backcross progeny, we found 3 strains with the recombinant type between them in 33 inbred strains tested. The results suggest that the Rtp-1 locus is very closely linked with the Mup-1 locus, which belongs to rat linkage group II. RTP-1 proteins strongly reacted with anti-MUP-1 A serum on agarose gel electrophoretograms.