A reproducible method for growing normal human bone marrow B-lineage colonies in agar is described. The clonogenic cells require a rich medium, Opti-MEM (GIBCO/BRL, Burlington, Ontario, Canada), and a source of T-cell-derived factors for growth. The conditions described appear to be limiting for the colony progenitor, suggesting assay clonality. Three novel methods that permit routine and rapid detection of these human B-cell colonies are also described. Colonies containing cells that secrete immunoglobulin are detected by plaquing and protein immunoblotting, while RNA transcripts can be detected by RNA colony blotting. The detection of more than one secreted immunoglobulin isotype or RNA species in a single colony can also be achieved. This B-cell colony assay and the associated detection methods will allow the further delineation of human B lymphopoiesis in both normal and disease states.