In order to clarify the blood group and blood protein composition of Holstein bulls in Japan, estimates of gene (allele or phenogroup) frequencies fornine blood group systems A, B, C, F, J, L, S, Z and R′ and for three bloodprotein loci Am-1, Tf and Ca were calculated by the maximum likelihood scoringmethed, from the 2, 959 bulls born in 1972-1979 and registered in Japaneseherdbooks.For calculation purposes, three FORTRAN 77comprograms II-1, II-2 and GFML were developed**. In the B system, 38 different phenogroupswere identified.The number of phenagroups with a frequency higher than 1%was 19 and the sum of their frequencies amoununed to 93.18%.The phenogroupswith a frequency higher than 2% were as follows: G₂Y₂E′₂ (19.52±0.56%), “-”(17.54±0.63%), J′K′O′(15.80±0.53%), BO1 (8.46±0.44%), O1 (4.31±0.35%), D′O′(3.51±0.24%), Y2 (3.19±0.36%), BO1Y2D′(2.69±022%), BG₂ KE′₂O′(2.69±0.22%), BO₁Y₂ (2.21±0.29%) and I2 (2.12±0.19%). Using the maximum likelihoodestimate of gene frequency and its standard error, the efficiency indicating thedegree of contribution where the genotype is deduced from thephenotype far eachgene in asystem or locus was calculated. From comparing the population (composed of bulls) in Japan with that (composed of cows and heifers) in the USA, genetic differences were recognized to exist between the two populations.<BR>The Holstein breed now accounts for most dairy cattle in Japan. This breed was first introduced into Japan from the USA and the Netherlands in 1889, and aregistration system in Japan was established in 1911. Today the Holstein cattlepopulation in Japan is virtually composed of individuals imported from the USA and Canada or their descendants born in Japan, except the minority imported from European countries (UK and the Netherlands). Now the numbers of bulls and cowsraised in Japan are approximately 1, 000 and 2, 100, 000, respectively.<BR>To calculate the estimates of gene (allele or phenogroup) frequencies and thei standard errors, we developed computer programs based on the method of maximum likelihood scoring of STEVENS.<BR>This paper (1) furnishes information on the gene frequencies for twelve blood group systems and blood protein loci of the Holstein bulls in Japan, which can help to study genetic differentiation among the populations, (2) investigates genes contributing to deduce the genotype from the phenotype in a system or locus, using the efficiency calculated from the maximum likelihood estimate of gene frequency and its standard error, and (3) compares blood group and blood protein composition of the population in Japan with that in USA..