Secreting and nonsecreting hybridoma populations derived from the murine hybridoma cell line 167.4G5.3 were each grown in batch culture in low serum and serum-free media. Under serum-free conditions, a secreting population gained on a predominantly nonsecreting population and competed with the existing antibody-deficient cells effectively. It was found that this competition was sensitive to state of inoculum and medium composition. We conclude that the competition between a secreting and nonsecreting, or more generally, a producing and nonproducing, population is important; the appearance of the latter may not be a significant setback in terms of expected product titer.