This report presents the characterization of three mouse monoclonal antibodies (mAb) reactive with the guinea-pig leucocyte common antigen (LCA); CD45 in the human nomenclature. One, IH-1, reacted with LCA on all leucocytes. The other two were more restricted: IH-2 recognized only the 220,000, 210,000 and 195,000 MW isoforms, and IH-4 the 220,000, 210,000 MW isoforms. Both IH-2 and IH-4 reacted with all B cells and all Kurloff cells [the putative guinea-pig natural killer (NK) cell]. IH-2, but not IH-4, reacted with monocytes and macrophages. Neither reacted with neutrophils. Most thymocytes expressed low levels of the IH-2 and IH-4 epitopes, with those expressing high levels located predominantly within the medulla. Most (90%) CD4+ T cells from newborn guinea-pigs expressed high levels of the IH-2 and IH-4 epitopes; this percentage decreased with age to 70% in 2-year-old animals. We have demonstrated that CD4+ T cells which express low levels of the IH-2 epitope also express low levels of the IH-4 epitope. CD8+ T cells can be divided into two subsets by IH-4 but not IH-2. The reactivities of IH-2 and IH-4 are remarkably similar to those of human anti-CD45RB and anti-CD45RA antibodies respectively. Analogies with man and other species suggest important functional differences for subpopulations of guinea-pig T lymphocytes defined by anti-CD45R antibodies.