To determine the persistence of immunocompetent B lymphocytes at the periphery, regardless of cell division, we have followed the decay of lipopolysaccharide (LPS)-reactive B cells in LPS-nonresponder, histocompatible hosts. Both the numbers of transferred cells and of those persisting in the recipients could be determine with precision by limiting dilution analysis of the various cell populations. Decay rates of 50% per day were determined. Because we can exclude immune elimination of donor cells and the numbers of transferred cells were too low to result in gross alterations of the physiologic turnover rates, we conclude that the majority of LPS-reactive B lymphocytes and a large part of the whole B cell compartment show this astonishingly high rate of decay. The method introduced here might prove useful in a variety of clonal assays, as it can detect cells present at a very low frequency.