SUMMARYNormal values for the percentages of lymphocytes carrying surface membrane -bound immunoglobulin, or resetting with sheep erythrocytes, ox erythrocytes coated with rabbit immunoglobulin G, or ox erythrocytes coated with rabbit immunoglobulin M and reacted with human serum partially de-complemented with zymozan, have been estimated for populations from human umbilical cord blood and the peripheral blood of babies aged one week and one month. These samples have been compared with values obtained from normal adult volunteers and eighteen newborn infants, measured during and after a suspected infective episode.The dynamic change in those lymphocytes from normal infants staining for both IgM and IgD is shown, demonstrating that the number of circulating cells of this typeapproaches adult levels by the age of one month. Falls in the levels of all three types of rosetting lymphocyte were demonstrated over the same period. A drop in the number of lymphocytes forming sheep erythrocyte rosettes, and particularly those binding C3b-coated ox erythrocytes, in those babies suspected of infection showed a correlation with acute septicaemia or viraemia.SPECULATIONThe fall in the sub-population of lymphocytes forming all three types of rosettes, with increasing age, is felt to reflect the sudden antigenic challenge experienced by the newborn infant after birth. In particular, the dramatic fall in those lymphocytes bearing immunoglobulins M and D may mark the selection of virgin antigen-sensitive B-lymphocytes in response to antigens. The fall in the proportion of lymphocytes binding to C3b-coated ox erythrocytes in infected infants may indicate either that the lymphocyte C3b receptor becomes blocked by free C3b released as a consequence of complement activation, or, more plausibly, that there are circulating antigen/antibody complexes bearing fixed C3b in these babies. Finally, it should be noted that a fall in any lymphocyte population in peripheral blood, particularly of cells binding sheep erythrocytes, may be more closely related to a segregation of those cells within the lymphoid system rather than a failure to express the relevant receptor.