Six patients with Philadelphia-positive (Ph1+) acute nonlymphocytic leukemia (ANLL) were studied by morphological, immunological, cytogenetic, and molecular techniques. Seven Ph1+ acute lymphocytic leukemia (ALL) cases were also studied for comparison. Three of ANLL cases were classified in M1, M2, and M4 groups of the FAB nomenclature, while the three other cases do not fit with any FAB subgroup and are described as M0. Immunophenotypical marker studies, double immunolabeling, and combined immunological and cytogenetic studies of metaphases showed that these ANLL expressed several lineage differentiation antigens. Rearrangements of immunoglobulin heavy chain gene (C mu) were detected in the six ANLL cases and in the seven ALL cases studied, as well as, in most cases, rearrangement of T cell receptor beta chain genes and/or T cell rearranging gamma genes. The results favored the assumption that the Ph1 translocation originated from a multipotent stem cell in Ph1+ ANLL. A common t(9;22) translocation was found in all cases, and additional chromosomal abnormalities were present in the six ANLL cases and in five of the seven ALL cases. Molecular studies of bcr gene configuration and c-abl transcription allowed two groups of Ph1+ ANLL to be distinguished. Three cases had bcr rearrangement and c-abl mRNA expression comparable to those reported in Ph1+ chronic myeloid leukemia, while three others had not detectable bcr rearrangement and a 7.2-7.5 kb c-abl mRNA. The existence of Ph1+ ALL with and without classical bcr rearrangement was confirmed.