A total of 179 Holstein cows from the Agriculture Canada Research Branch herd at Ottawa and 271 progeny-tested Canadian Holstein bulls were typed for 37 lymphocyte antigens. Each antigen appears to be controlled by a distinct, codominant allele of the bovine lymphocyte antigen (BoLA)-A locus, which is a class I locus of the bovine major histocompatibility system. Only 10 and 16 alleles were present in Holstein cows and bulls, respectively. The association between bovine lymphocyte antigens and economically important traits was examined in gene substitution models. Separate models were used for the cow and bull data. The substitution of the W6.1 allele for the W10 allele was associated with increased protein yield in the milk in both analyses. The results were also consistent with a previous report that the presence of the W10 allele was associated with increased fat percentage compared with some, but not all, of the other alleles. However, more research is necessary to confirm these findings and to determine the biological mechanisms underlying these associations.