Contact hypersensitivity to picryl chloride in the mouse is proposed as a valuable tool for the in vivo immunotoxicological testing of new compounds. Reproducibility was found to be excellent. Results obtained in the present work with chlorpromazine, cyclophosphamide, diazepam, haloperidol, hydrocortisone, promethazine and lead acetate, nickel chloride and selenium were similar to those previously reported regarding the effects of these compounds on cell-mediated immunity.