This experiment was carried out to estimate phenotypes and gene frequencies in genetic variant of transferrin(tf) system in serum proteins from total 137 individuals racing horses reared in Korea by means of starch gel electrophoresis, and thus to get fundamental data for the improvement of racing horses by pedigree registry using blood type. The results obtained are summarized as follows: 1. At the present time, serum Tf types in horses are controlled by 11 alleles, named Tf^A(Tf^D), Tf^B(Tf^(F1), Tf^(F2) and Tf^(F3)), Tf^G, Tf^C(Tf^(H1) and Tf^(H2)), Tf^J, Tf^M, Tf^D(Tf^O), and Tf^E(Tf^R). In Korean racing horse population, however, Tf types were shown to be controlled by five codominant autosomal alleles, Tf^A, Tf^B Tf^C, Tf^D and Tf^E, and alleles Tf^(F3), Tf^G, Tf^J and Tf^M were not recognized. 2. In the Tf system 13 phenotypes(5 homozygote and 8 heterozygote), AA, BB, CC, DD, EE, AB, AD, AE, BC, BE, CD, CE and DE were classified. However, heterozygous Tf phenotype AC was not found. The distribution of Tf phenotypes was Tf BB type 33.58%. Tf AB type 28.47% and Tf AA type 10.95%. All the other phenotypes were lower than 10%. The expected numbers of phenotypes were in good agreement with the observed numbers 3. The frequencies of the five alleles Tf^A, Tf^B, Tf^C, Tf^D and Tf^E were 0.296, 0.544, 0.029, 0.073 and 0.058, respectively. Therefore, gene frequencies indicate that Tf^B was the most common allele and, secondarily, Tf^A gene. The Tf^C, Tf^D and Tf^E showed low gene frequency. These results were stikingly similar to the data reported in Thoroughbred and Anglo-Arab breeds.