In order to further understand the developmental aspects of B-1 cells, we characterized the ontogeny of this B cell population in the spleen and peritoneal cavity of BALB/c mice. Although there are B-1 cells in the spleen within the first 1-3 weeks after birth, they do not at any stage represent the majority of splenic B cells. Splenic B-1 cells reach peak levels at ∼9 days after birth. The mesenterlc lining that covers the small intestine of 7-day-old mice contains a population of IgM+ B cells, while at the same age, there are few lymphold cells in the peritoneal cavity. Between 7 and 8 days after birth there is an influx of B cells into the peritoneal cavity. At 8 days, the first detectable peritoneal B cells appear to be of the B-1 type based on expression of IL-5 receptor and CD5. However, these peritoneal B-1 cells do not express Mac-1. This antigen Is not expressed by the majority of peritoneal B-1 calls until 3 weeks. This study indicates that the majority of early splenic B cells are not B-1 cells and it suggests that the mesenterlc tissues surrounding the gut contain B lymphocytes which traffic into the peritoneal cavity where they then reside. © 1994 Oxford University Press.