Foetal lambs were immunized orally 6-15 days before birth by introducing horse spleen ferritin into the amniotic fluid. Immunized and non-immunized lambs were killed at birth, usually before they had suckled, blood and intestinal contents were collected and single cell suspensions were prepared from spleen, mesenteric lymph nodes and jejunum. Specific antibody was detected in serum and intestinal contents of all immunized lambs which had not suckled. Specific antibody was usually not detected in samples from non-immunized lambs. In immunized lambs antibody activity in serum was associated with IgM and in intestinal contents with IgA and IgM. In agreement with these findings, the levels of IgM and IgA in serum and intestinal contents of immunized lambs were relatively high. Generally, immunoglobulins were not detected in samples from non-immunized lambs. Relatively high proportions of cells secreting specific antibody were present in the tissues of immunized but not non-immunized lambs. In the spleen most of the cells were secreting IgM antibody, in mesenteris lymph nodes IgM cells predominated and small numbers of IgA cells were detected, and in the jejunum approximately equal numbers of IgA and IgM cells were secreting specific antibody.