Ly-1 B cells in mouse show numerous phenotypic and functional features that distinguish them from the bulk of IgDhigh/Ly-1- B cells. Their association with autoantibody production and the presence of Ly-1 on a group of murine B lymphomas that also exhibit certain specificities enriched in the normal population has stimulated continuing interest in this population. We have taken two approaches in our investigations of these cells: 1) defining the origins of Ly-1 B cells (the "lineage question"); and 2) studying the expression of particular specificities and associated immunoglobulin V genes enriched in this population. In this review we present the experimental background that supports our current understanding of Ly-1 B cells as the remnant of a fetal B cell differentiation pathway and suggest that the selection of cells from this fetal/neonatal population into the adult long-lived pool results in the over-expression of certain germline-encoded autoreactivities, such as antibody to bromelain-treated mouse red blood cells and intact thymocytes.