In recent years there has been abundant interest in the properties and significance of the CD5+B cell subpopulation, fuelled by evidence that CD5+B cells play a central role in autoimmunity. Much of the current dogma about this cell subset is derived from studies in the mouse. On the basis of adoptive transfer experiments, murine CD5+B cells have been suggested to constitute a lineage distinct from“conventional”B cells, arising from progenitors which are present in fetal liver and omentum, but not in adult bone marrow. Moreover, a third lineage has been proposed, consisting of B cells which lack the CD5 antigen but which resemble in other respects CD5+B cells. These subpopulations are now termed Bla and Bib cells respectively, in contrast to the conventional B2 subpopulation. In their review, Kasaian and Casali discuss the role of human CD5+B cells in autoimmunity, and provide evidence for a subpopulation which lacks CD5 but can be distinguished by its lower than average expression of isoforms of leucocyte common antigen identified by CD45RA antibody. On the basis of their expression of CD5 mRNA and their apparent commitment to the production of natural antibodies, it is suggested that these may be the human equivalent of the mouse Bib cells.