Mice produced reaginic antibody within 1 wk of painting with the contact sensitizing agent picryl chloride. The titers, measured by passive cutaneous anaphylaxis in rats, increased after repeated applications of picryl chloride. In contrast, serum agglutinins did not increase after two applications of picryl chloride. Reagin was also elicited by another contact sensitizing agent, oxazolone. Some strain variation of the response to picryl chloride was found, with CBA mice being good responders and BALB/c and C57BL/10 mice being poor responders.