Two groups of pregnant guinea pigs were sensitized to ovalbumin (OA) by injecting the antigen, incorporated in Freund's incomplete or complete adjuvant, via the IM or foot pad route, respectively. Inoculation of OA into the lumen of the mammary glands during subsequent lactation elicited an influx of leukocytes into the milk only in guinea pigs exhibiting OA-specific delayed-type cutaneous hypersensitivity (DTCH); ie, only in the animals that previously were given OA in Freund's complete adjuvant. Lactating female guinea pigs, passively sensitized to OA by immune serum obtained from donors that exhibited specific DTCH, did not respond top intramammary inoculation of OA. Recipients of peritoneal exudate cells, obtained from syngeneic donors that exhibited DTCH to OA, in turn exhibited DTCH and responded to intramammary inoculation of OA with a transient release of leukocytes into the milk. Emigration of leukocytes into the milk, elicited by antigen in specifically sensitized hosts, appeared to be a local expression of a general state of lymphocyte-dependent hypersensitivity.