A 56-yr-old woman with acute lymphocytic leukemia showed the presence of a Philadelphia chromosome in 90% of the bone marrow metaphases and in 10% of the peripheral blood metaphases. Part of the long arm of a G22 chromsome was translocated to the long arm of a C9 chromosome. A second cytogenetic abnormality was found in chromosome 7. Monosomy 7 was found in 60% of the marrow and in 20% of the peripheral blood metaphases. Chromosome 7q- was also found in a small percentage of the metaphases examined. Three months later, when the patient was in partial remission, only 10% of the marrow cells showed chromosome 7 monosomy and a Ph1 chromosome. During complete remission, no chromosomal abnormalities were found, except for a high breakage rate. The finding of a Ph1 chromosome in acute lymphocytic leukemia indicates that different precursors, both granulocytic and lymphocytic, may be involved in the Ph1 process.