The ontogeny of the murine intestinal B-cell compartment before and after weaning was studied by quantitative analysis of immunoglobulin-secreting cells (Ig-SC) in the small intestine (SI). Before weaning, few Ig-SC were detected in the SI, whereas spleen and bone marrow already contained many Ig-SC. The number of Ig-SC in the SI started to increase immediately after weaning. Comparing early-weaned mice with non-weaned mice of the same age clearly demonstrated that weaning brought on the development of Ig-SC in the SI. The influence of a gut flora on the number of Ig-SC in the SI was examined by comparing the number of Ig-SC in the SI of conventionally housed, specific pathogen free (SPF) and germ-free mice. A bacterial flora was apparently needed for the normal development of Ig-SC in the SI. Comparing mice containing an aerobic Gram-negative bacterial flora with mice containing only an anaerobic Gram-positive bacterial flora demonstrated that the type of bacterial flora is relatively unimportant. No evidence was found that circulating maternal antibodies suppressed the development of the "spontaneous" intestinal and systemic B cell response. The results show that bacterial colonization of the intestine plays a pivotal role in the development of the Ig-SC compartment in the SI.