Using segregation and population analysis of serological data prepared in our laboratory, we have tentatively defined five OLA antigenic specificities: CO1.1, CO1.2, CO2.1 and CO2.2, whose genetic control would fit a model of two class I OLA loci (CO1.1 and CO1.2 first locus, CO2.1 and CO2.2 second locus, and CO3.1, that could represent a subtype of CO2.2. Alloantisera were obtained from Spanish Merino sheep by intrafamilial cross-immunization and from serum of primiparous and multiparous ewes. Screening of alloantisera for antilymphocyte reactivity was carried out by a two-step dye exclusion microcytotoxicity test against 216 animals belonging to different local breeds: Spanish Merino, Merino Landschaft, Manchega and Churra. Lymphocyte antigen specificities were characterized by cluster and correlation analysis, as well as family studies. Comparison of CO alloantisera with reference reagents from Scotland (CA reagents) showed an excellent correlation between CO1.1 and CA2, and CO2.1 and CA5. The reagents described can be used in various breeds, a characteristic important for their future applications.