Polymorphism of bovine MHC (BoLA) class I molecules has been characterised mainly by serology. The use of alloantisera has led to the identification of 50 different serological specificities, most of which behave as alleles of a single highly polymorphic locus. However, molecular biological and biochemical studies suggest that there are at least two class I genes expressed from any one haplotype. The actual number of expressed MHC class I genes in cattle is as yet unknown. The aim of this study was to isolate different functional class I genes from a heterozygous animal, class I typed at BoLA-A10/A11. In order to investigate the expression and function of individual class I products, transfection and characterisation of a series of bovine class I genomic and cDNA clones isolated from this animal were performed. From a group of fifteen bovine class I genomic clones, five different clones (1.3, 4.2, 15.2, 17.3x and 19.1), characterised by their exon 2 sequences and by restriction analysis of an amplified 3.2kb class I gene fragment, were transfected into mouse L cells. Of the five clones transfected, only 19.1 and 4.2 showed expression at the L-cell surface by using a murine monoclonal antibody specific for a non-polymorphic determinant on bovine MHC class I (IL-A88) in flow cytometry. The phage clone 19.1 which had been previously shown to express a bovine class I molecule with A11 specificity, served as a control in these transfection experiments. The transfectants obtained from both expressing clones were characterised serologically, biochemically and by cellular assays and both were found to encode A11 serological specificity. Interestingly, the products of the 19.1 and 4.2 transfected class I genes were indistinguishable by isoelectric focusing, despite having clearly different nucleotide sequences.