The immunoglobulin mu heavy chain gene of mouse hybridoma cells is expressed in two forms, microseconds and microns, differing in their use of 3' exons. As for many other mammalian genes, mutations in the mu gene which prematurely terminate translation often have the effect of reducing the amount of these mu RNAs. To test the generality of this relationship, we selected mutant hybridoma cell lines defective in IgM production and searched both for translation termination mutations which do not reduce the amount of mu RNA as well as for mutants which show the more commonly observed reduction in mu RNA. As observed previously, the amount of microseconds RNA is normal in mutants terminating in the C mu 4 exon; by contrast the amount of microns RNA is reduced in these mutants, indicating that the effect of the mutation is influenced by some feature near the 3' end of the RNA. Mutations terminating translation in other C region exons have a graded effect on RNA content, ranging from 10% the normal level for termination in the C mu 3 exon down to 1% for termination in the C mu 2 exon. By contrast, a mutant cell line terminating in the leader exon contained 25% the normal amount of mu RNA, suggesting that translation past some point might be required to fully engage the RNA degradation process.