CD2 is a surface marker of T cells and NK cells, and is not normally expressed on human myeloid cells, but is found on a significant minority of cases of acute myeloid leukemia (AML). Molecular studies were performed on bone marrow myeloblasts from two patients with CD2 surface positive AML. No abnormality of the CD2 gene was detected on Southern blot analysis. On Northern blots, CD2 mRNA of normal size was present. The CD2 gene contained a site which was unmethylated, consistent with active transcription, in a CD2 positive AML case, and in a CD2 positive T cell line, but methylated in CD2 negative AML cells. The evidence does not support the hypothesis that inappropriate surface expression of lineage markers is due to leukemia-related genetic changes, such as amplification or rearrangement, of the CD2 gene itself. Rather, the results are consistent with the hypothesis that mixed lineage leukaemia arises from rare normal bone marrow progenitors with multilineage phenotypes.