Experiments were performed on human B-lymphocytes in paired samples of milk and blood from the same patient to determine the numbers and percentages of lymphocytes with complement receptors, utilizing the erythrocyte-antibody-complement (EAC) rosette assay. Studies on nine paired samples show the numbers, as well as the percentages, of complement-reactive cells (CRCs) in milk are significantly lower than in blood. Milk lymphocytes were cultured in media containing serum for a 24 hour period to determine if the observed lower numbers of CRCs in milk were due to saturation of the lymphocyte receptors with free complement, as well as to determine if the culture of the cells in nutrient media might result in the reformation of shed complement receptors. Similarly, peripheral blood lymphocytes were incubated in milk supernatant to determine if free complement or other factors in milk might interfere with the EAC assay. These studies support previous findings from this laboratory that B-lymphocytes in milk represent a subpopulation of cells different from those in peripheral blood. The paucity of B-lymphocytes bearing complement receptors in milk may be due to the following: (1) B-lymphocytes in milk may be plasmablasts and/or antibody-producing plasma cells--both known to lack complement receptors; (2) selective processes operating at the mammary alveolar epithelium allowing transit of certain subclasses of maternal immunoglobulins may also allow transepithelial passage of B-lymphocytes with surface immunoglobulins, but no complement receptors. These possibilities are currently being studied.