Abstract Flow cytometry has been utilized to evaluate the stability of antibody production by unstable subclones of a human × human × mouse heterohybridoma. Heterogeneity of cell-associated immunoglobulin heavy chain expression was demonstrated in different subclones and an increased frequency of cells containing low levels of heavy chain was found to correlate with low antibody productivity. However, the majority of cells were not completely devoid of heavy chains suggesting that the genetic information for the γ chain was not lost. In contrast, the gene encoding the κ light chain was shown to be absent from the subclones expressing low levels of heavy chain and these subclones also contained substantially reduced levels of heavy chain mRNA, suggesting that the production of this protein was controlled at the level of transcription or mRNA stability. In conclusion, the correlation of staining intensity as observed by flow cytometry and antibody productivity makes flow cytometry a suitable technique for the rapid evaluation of heterohybridoma cell lines used for antibody production.