Human beta-globin genes introduced into mouse erythroleukemia (MEL) cells by DNA cotransformation are correctly regulated when erythroid cell differentiation is induced by dimethylsulfoxide (DMSO). In contrast, cloned human alpha-globin genes are efficiently transcribed in MEL cells prior to induction, and no increase in the level of alpha-globin mRNA is observed when the cells differentiate. These observations suggest that the mechanisms by which alpha- and beta-globin genes are activated during erythroid cell differentiation are fundamentally different. Analysis of the transcription of hybrid human alpha/beta-globin genes in MEL cells revealed that the sequences responsible for differences in transcription of the intact alpha- and beta-globin genes are located on the 3' side of the mRNA capping site of the two genes, suggesting that cis-acting regulatory sequences are located within the structural genes.