ABSTRACT:The leukocytic response of the mammary gland in the locally vaccinated guinea pigs to challenge with specific antigen during lactation was investigated. The response was measured by enumerating cells in milk collected at various times prior to and following antigenic challenge. A significant leukocytosis was observed in milk from vaccinated animals. The maximum cellular response by vaccinated animals was observed at 16 h to 30 h after challenge. The majority of leukocytes collected at that time did not form EA rosettes. Differential cell counts showed that the polymorphonuclear leukocytes were the major cell type until 30 h postchallenge while the mononuclear leukocytes predominated later. The delayed cellular response to challenge and the predominancy of leukocyte type at various times after challenge are discussed. It is proposed that the leukocyte response of the mammary glands in vaccinated animals was a cell‐mediated immune reactio