Flow cytometry was used in combination with immunofluorescent staining for intracellular and surface-associated antibody contents to identify a significant nonproducer cell fraction in a murine hybridoma cell line that had shown a decline in monoclonal antibody productivity with passaging. Viable producer cells stained for surface-associated antibody content were isolated by cell sorting on the basis of surface fluorescence intensity. The fraction of nonproducers was initially reduced from 85% to 20%. Sorting a second time, after these cells were cultivated for 2 weeks, further reduced the fraction of nonproducers to less than 4%. The stability of this purified producer hybridoma cell line during passaging and after a freeze-thaw cycle was investigated. This cell line was found to be highly unstable. A simple batch-growth model simulation was used as follows : (i) to demonstrate that nonproducers appear in hybridoma cell lines after a rare, random mutation event that results in the loss of the heavy-chain gene and/or light-chain gene expression; (ii) to show that a high rate of conversion of producer hybridomas to nonproducer hybridomas cannot entirely explain the population dynamics; (iii) to estimate the rate of conversion of producers to nonproducers to be 8.7 x 10(-5) h(-1); (iv) to show that, for this conversion rate, the nonproducer cells’ specific growth rates need only be 9% higher than those of the producer cells to dominate the hybridoma culture after only 25 passages; and (v) to predict that producer cells are preferentially lost during eel freezing and thawing procedures. The data suggest that particularly unstable cell lines should be analyzed and purified frequently to prevent overgrowth by nonproducers.