The antigen-induced DNA synthetic response of a number of different thymocyte populations was studied in the spleens of lethally irradiated recipients by incorporation of 125I 5-iodo-2-deoxyuridine. The response of certain cell combinations was not only less than the sum of the two responding alone but less than one of the individual cell populations in the combination. The following combinations had this effect: 1) “Educated” and normal thymocytes < “educated” alone in the response to sheep red cells (SRBC); 2) parental and F1 thymocytes < parental alone in F1 hosts; 3) cortisone resistant (in a low dose) and normal thymocytes < normal alone in the response to SRBC. Thus, thymocytes are capable of suppressing the antigen-induced response of other thymocytes without the mediation of B cells or their product (i.e., antibody).