The SCL gene encodes a member of the 'helix-loop-helix' family of DNA binding regulatory proteins. It is transcriptionally activated in some cases of T-cell acute lymphoblastic leukaemia by a reciprocal translocation involving the T cell receptor delta locus. In order to gain insight into the normal functions of SCL we have studied SCL mRNA levels in human and murine haemopoietic cell lines and normal tissues. We have observed high levels of SCL mRNA in all human and murine erythroid cell lines examined. Foetal and adult normal haemopoietic cell populations rich in erythroid precursors also expressed high levels of SCL mRNA. Our results suggest a previously unexpected role for SCL in the regulation of erythropoiesis.