We show further differences between two clinically related entities, T cell acute lymphoblastic leukemia (T-ALL) and lymphoblastic lymphoma (T-LL), by using several monoclonal antibodies (mAb) reacting either with constant or variable regions of T cell receptors (TcR) alpha beta and gamma delta or with various CD molecules. We analysed a panel of 15 T-ALL and 15 T-LL selected for their cell surface expression of the CD3 molecules. The results indicate that TcR gamma delta is more frequently used than TcR alpha beta in T-ALL (10 out of the 15 patients tested). This is in contrast to the results obtained with T-LL where the vast majority expressed TcR alpha beta (13 out of the 15 patients). These findings suggest that the leukemic cells could have a different origin in these two diseases. In addition analysis of TcR variable regions expressed by the leukemic blasts showed that in most cases they had rearranged functional V delta 1 gene segments (8 out of 11 patients) whereas in a unique case V delta 2 gene segment was used. Taken together these results and those indicating that T-ALL cell coexpress the CD1a,b and c molecules strengthen the possibility that even though these leukemic cells express the CD3-TcR complex at their cell surface their normal counterparts are not found in peripheral blood.