The development of B-lineage cells requires a series of complex interactions with hemopoietic stromal cell elements during the generative phase, and with antigen and T lymphocytes during the subsequent proliferative/differentiative phases in lymphoid tissues. Recent advances have been made in defining developmental changes in structure and assembly of the antigen receptors and in identifying protein kinases involved in signal transduction via these receptors. The mechanism of T-cell help has also come into much clearer focus through elucidation of the interaction between CD40 on B cells and the CD40 ligand on activated T cells. Finally, progress has been made with the recent identification of defects in a cytoplasmic protein tyrosine kinase and in the CD40 ligand as causes of two B-cell immunodeficiencies in man. © 1993.