Models of B-cell development in the immune system suggest that only those immature B cells in the bone marrow that undergo receptor editing express V (D)J -recombination-activating genes (RAGs)^,,. Here we investigate the regulation of RAG expression in transgenic mice carrying a bacterial artificial chromosome that encodes a green fluorescent protein reporter instead of RAG2 . We find that the reporter is expressed in all immature B cells in the bone marrow and spleen. Endogenous RAG messenger RNA is expressed in immature B cells in bone marrow and spleen and decreases by two orders of magnitude as they acquire higher levels of surface immunoglobulin M (IgM). Once RAG expression is stopped it is not re-induced during immune responses. Our findings may help to reconcile a series of apparently contradictory observations, and suggest a new model for the mechanisms that regulate allelic exclusion, receptor editing and tolerance.