Previous studies in sheep indicate that a combination of intraperitoneal and intramammary immunisation with ovalbumin results in a population of anti-ovalbumin-containing plasma cells of the IgG1 isotype in the immunised mammary gland. These cells are not present in the unimmunised contralateral gland. The studies reported here were undertaken to determine the source of these cells. The failure of chronic intestinal lymphatic drainage or either mesenteric or supramammary lymphadenectomy to abrogate this response was interpreted as evidence against their origin from gut-associated or mammary-associated lymphoid tissue. It is concluded that the success of prior intraperitoneal immunisation in stimulating an enhanced IgG1-specific response to local antigen in the mammary gland probably results from recruitment of cells from systemic lymphoid tissue primed by intraperitoneal immunisation.