The production and characterization of eight monoclonal antibodies (MAbs) against surface markers of guinea pig T-cells is reported. MAbs CT5 and CT7 define putative pan-T-cell markers. CT5, however, also reacts with the B-cell leukemic line L2C. MAb CT6 is reactive with less than 30% of peripheral T-cells. MAbs CT1, 2, 3, 4, and 8 are reactive with lymphocytes, but not with germinal center B-cells. In addition to the CT's, a MAb (CI-13.1) has been prepared that reacts with an Ia-like antigen on cells of strain 13 and outbred guinea pigs, but not with cells of strain 2 animals. CI-13.1 cross-reacts with human tissue sections: About 30% of the OKIa-positive dendritic cells in the human dermis are recognized by CI-13.1. In the course of production and characterization, various binding assays and an immunohistological method were used for determining the antibody specificity. Immunohistological screening was found to be the most informative method.