MHC-class II antigens on canine lymphoid cells were detected with monoclonal antibodies (Mabs). On the basis of reactivity with resting or activated T lymphocytes two groups of Mabs were reported earlier. Non-activated T lymphocytes expressed MHC-class II antigens recognized by the first group whereas the second group recognized only MHC-class II antigens on activated T lymphocytes. In this study we analysed the reactivity pattern of both groups of Mabs with purified canine B lymphocytes. One- and two-colour immunoflowcytometric analysis, immunoprecipitation, immunohistology and MLC inhibition were performed. The Mabs which only reacted with activated T lymphocytes appeared to stain also purified sIg+ lymph node cells. Two-colour fluorescence and immunohistology confirmed the reactivity of these Mabs with B lymphocytes. Immunoprecipitation showed the bimolecular structure of these antigens. It was concluded that two subsets of MHC-class II antigens can be detected on canine lymphoid cells. One subset showed an aberrant distribution being expressed on both nonactivated and activated T and on B lymphocytes. The second subset of MHC-class II antigens could only be detected on B lymphocytes and activated T lymphocytes, a distribution pattern similar to that found in most other species, for instance man.