Here we report the case of a 7-month-old boy who presented with biphenotypic acute leukemia, but with leukemia cells of B-cell phenotype present at the time of relapse. Two cell lines were derived from bone marrow specimens obtained at relapse, and immunophenotyping and analysis of antigen receptor gene configuration revealed concordance between the patient's leukemic cells and the cell lines. Cell line PER-377 shows a new chromosomal abnormality, t(2;13)(p12;q34), a molecular rearrangement at chromosome band 11q23 in the absence of a cytogenetically detectable abnormality of this band, and deletion of the genes for IGK.