Guinea pigs were vaccinated intramammarly (IMM) with attenuated Mycobacterium bovis (BCG) and challenged intramammarly and intradermally with tuberculin. A significant (p less than 0.05) milk leukocytosis, consisting primarily of polymorphonuclear leukocytes, occurred from 6-30 hr after challenge with tuberculin. Intradermal challenge with tuberculin produced typical delayed-type hypersensitivity cutaneous reaction in these animals. Normal guinea pigs were adoptively sensitized with lymphoid cells from the inguinal lymph nodes or spleen of BCG-vaccinated animals and subsequently challenged intramammarly and intradermally with tuberculin. The mammary and dermal responses of the lymphoid cell recipients were similar to, but less pronounced, than those in the actively immunized animals. The responses in the recipients of lymphocytes from donors injected with tuberculin 18 hr prior to cell collection were greater than those from injected donors. Guinea pigs that were injected intraperitoneally with serum from BCG-vaccinated donors did not express significant dermal or mammary responses to subsequent challenge with tuberculin. It was proposed that the milk leukocytosis was mediated by sensitized lymphocytes stimulated by IMM challenge with tuberculin.