The response of PBMC collected from cattle immunized with killed Cowdria to MAP1, MAP1-B, MAP2 and to a lysate of Cowdria was characterized in vitro. CD4+ T lymphocytes responded strongly to Cowdria lysates while B cells and yb T cells proliferated in response to recombinant proteins. Short-term cell lines derived from PBMC using Cowdria lysates were more than 90% CD4+ and did not respond to any of the recombinant proteins. Restimulation of PBMC with MAP1, MAP1-B, and MAP2 proved difficult, and cultures usually stopped proliferating after 2 to 3 weeks. Only one cell line could be generated using MAP1 and it was composed of 85% yb T cells. This study indicates that in animals immunized with killed Cowdria, proteins other than the serologically immunodominant MAP1 and MAP2 are strongly immunogenic for T lymphocytes.