An in vitro assay which measures specific antigen-induced proliferation of primed rabbit lymph node and peripheral blood cells is described. This response was found to be mediated by T cells, since it could be obtained with nylon-wool passed cells and cells which do not adhere to anti-Ig-coated plastic plates. The proliferative response was found to be highly antigen-specific and restricted to the draining lymph node if assessed up to 15 days post-priming. Purified T cells required antigen-pulsed accessory cells to proliferate. The proliferative response of peripheral blood lymphocytes was found to be low unless the lymphocytes were fractionated on insolubilized histamine. The histamine-adherent cells could suppress the above peripheral blood response, implying a certain regulatory role of suppressive cells in the periphery. The ease, reproducibility and specificity of the assay provides a simple method to evaluate the characteristics of a T-cell response in the rabbit.