It is recognized that the Ph1 chromosome anomaly is characteristic of chronic granulocytic leukemia. It has been shown however, to occur rarely in other myeloproliferative diseases. We have reported a 53-year-old roentgenologist with acute lymphocytic leukemia who showed a high percentage of Ph1 chromosomes in his marrow cells. The possible role of radiation exposure in the production of the abnormal chromosome in this patient is discussed. The data presented is evidence against the specificity of the Ph1 chromosome for myeloproliferative disorders.