Dimethyl sulfoxide-induced Friend cells were labeled for periods of 5-60 min. The denatured RNA was fractionated by sucrose gradient centrifugation and the distribution of alpha- and beta-globin-specific [(3)H]RNA was determined by hybridization to hybrid plasmids containing mouse alpha- and beta-globin DNA, respectively. After 5 min of labeling, a 15S peak of beta-globin-specific (but not alpha-globin-specific) [(3)H]RNA was detected, next to an equal amount of 10S beta-globin [(3)H]RNA. With increasing periods of labeling, the amount of 15S beta-globin [(3)H]RNA remained constant but the amount 10S beta-globin [(3)H]RNA increased steadily. alpha-Globin-specific [(3)H]RNA sedimented at 11 S after 5 min of labeling and at 9.5 S after longer labeling periods. Analysis of 15S globin-specific [(3)H]RNA purified by the poly(dC)-cDNA method [Curtis, P. J. & Weissmann, C. (1976) J. Mol. Biol. 106, 1061-1075] showed oligonucleotides characteristic of beta-globin mRNA but not of alpha-globin mRNA, as well as about 20 new oligonucleotides. Our results suggest that 10S beta-globin mRNA arises via a 15S precursor that has a half-life of 5 min or less; 9.5S alpha-globin mRNA may be derived from an 11S precursor.