Sixteen murine monoclonal antibodies (MAb), reactive with HLA-DR, DR + DP or DR + DQ, were tested, using indirect immunocytofluorescence, for their reactivity with peripheral blood mononuclear cells (PBMC) from the dog, cat, guinea pig, sheep, rabbit and rat. In addition, the MAb were evaluated for inhibitory activity in the canine mixed lymphocyte culture (MLC). Fourteen of 16 MAb reacted with canine PBMC. There was a greater tendency for DR + DP reactive MAb to inhibit canine MLC and subsequently react with PBMC of the guinea pig, sheep, and, cat. MAb failing to block the canine MLC were generally nonreactive with guinea pig PBMC (7 of 9 nonreactive) suggesting the guinea pig may be a useful model to study the functional relevance of specific Ia molecules. One MAb, H81.98.21 (reactive with HLA-DR) blocked canine MLC and reacted with PBMC from all species tested suggesting the determinant it recognized to be very well conserved in nature.