Using cDNA hybridization, mouse globin RNA sequences can be detected in the nuclear and cytoplasmic RNA not only from erythroid cells (fetal liver and reticulocytes), but also in low amounts in non-erythroid tissues (adult brain and liver, and cultured lymphoma, untransformed, and transformed fibroblast cell lines). For the nuclear RNAs, melting curve, density, and size determinations on the hybrids confirmed the presence of an RNA species indistinguishable by cDNA hybridization from reticulocyte globin mRNA. Thus the genes coding for alpha- and beta-globin, proteins thought to be found only in a restricted range of differentiated cell types, may nonetheless be transcribed at very low levels in all mouse cells. The proportion of the globin RNA sequences in the nuclear RNA that are found associated with poly(A) sequences does not vary markedly from one tissue or cell line to another. In erythroid cells, however, the proportion of the cellular globin RNA sequences that are found in the cytoplasm is very much higher than in non-erythroid cells. This suggests the existence of messenger-specific nuclear post-transcriptional control mechanisms which do not use poly(A) as a signal, and which allow the cytoplasmic accumulation of globin mRNA only in erythroid tissues.