A leukemia line KOPN30bi was established from a patient with acute lymphocytic leukemia (ALL) and Philadelphia (Ph) chromosome. The clonal rearrangement of the immunoglobulin heavy chain (IgH) gene and the expression of the P190 type BCR/ABL chimeric transcript were shown to be identical between KOPN30bi and the predominant clone (S1) in the blast cell population from which KOPN30bi was established, indicating that they are of the same clonal origin. Studies of the T-cell antigen receptor (TCR) gene configuration including the TCR beta, gamma, and delta loci showed that none of them was identical between KOPN30bi and S1. The TCR delta region was rearranged on both of the alleles in KOPN30bi and was deleted on both alleles in S1 indicating that KOPN30bi was not derived from S1. Polymerase chain reaction analysis, using an oligonucleotide probe corresponding to the N region sequence of the V gamma-J gamma juncture of KOPN30bi, indicated that only 0.1% of the blast cells corresponded to KOPN30bi. Thus molecular diversification of dominant subclones in vivo and in vitro was shown in Ph-positive ALL.