The relationship between karyotype and expression of the T-cell receptor (TCR) proteins was examined in I9 patients with T-lineage acute lymphoblastic leukemia (T-ALL). All patients expressed CD3 molecules in the cytoplasm or on the cell membrane. Patients were classified according t o TCR expression thus: no TCR expression (TCR -), six cases; cytoplasmic expression of TCR beta chain (cTCRB) only, six cases; membrane expression of TCR alpha and beta chains (mTCRAB), five cases; membrane expression of TCR gamma and delta (mTCRGD), two cases. A chromosomally abnormal clone was detected in I5 cases. The most common site of chromosomal change was at 14q I I (seven cases), the chromosomal band t o which TCRA and JCRD have been mapped; as a deletion (two cases); o r as a translocation with reciprocal breakpoints in bands containing the TCRG (7p IS); JCRB (7q35); or putative oncogenes HOXl I ( I Oq24), RBTNZ (I I p I3), o r MYC (8q24) genes. Breakpoints were also seen in 6q (three cases), 9p (two cases), o r I lq23 (two cases). The following observations were made: All four chromosomally normal cases lacked TCR expression (TCR-). Breakpoints at 14ql I were found in one of six TCR-cases, four of six cTCRB cases, and two of five mTCRAB cases. Abnormalities of 6q and of 9p were seen only in cases with full TCR expression (mTCRAB or mTCRGD). Genes Chrorn Cancer 4:4 1-45 ( I 992).