The nuclear enzyme terminal deoxynucleotidyl transferase (TdT) is thought to contribute to the diversity of certain immunoglobulin and T cell receptor gene rearrangements through the addition of random nucleotides at their variable (V)-joining (J) region junctions. An acute lymphoblastic leukemia (ALL) with an immature T cell phenotype (CD7+, CD5+, CD1+/-, CD2+/-, CD3-, CD4-, CD8-) was found to be TdT+ with germline immunoglobulin heavy chain, T cell receptor beta chain, and T cell gamma chain genes. The data indicate that TdT expression can precede T gamma and T beta rearrangement during T lymphoid ontogeny consistent with its proposed association with the T cell receptor rearrangement process. Southern analysis of certain cases of T-ALL may not result in the detection of a monoclonal population of cells.