We studied the responses of unpurified mouse thymocytes and of peanut agglutinin-negative mouse thymocytes to the pI 7.3 form of rabbit Interleukin-1. We found that small increases of temperature strongly enhanced the mitogenic effect of this form of IL-1, and that the apparent temperature optimum was 37 degrees C. In both these respects the behavior of the pI 7.3 IL-1 resembled the previously described behavior of pI 5.0 IL-1. We suspected that the low apparent temperature optimum for IL-1 action was due to inadequate pH control by the bicarbonate-buffered medium. Experiments showed that small decreases in medium pH strongly inhibited the mitogenic action of both forms of IL-1. Furthermore, if thymocytes were stimulated with either form of IL-1 in strongly buffered media, the temperature optimum was at least 39 degrees C. The pI 7.3 and pI 5.0 forms of IL-1 are known to differ both biochemically and immunologically. Our experimental discovery that their temperature sensitivities are much the same suggests that temperature sensitivity is a property of the T cell rather than of the IL-1 molecules themselves.