The chromosomes of a cell line (NALM-1) derived from the leukocytes of a patient with chronic myelocytic leukemia (CML) were examined with several banding techniques. The modal chromosome number was 46 and the cells contained a Philadelphia chromosome (Ph1), due to the standard translocation of the missing segment of the long arm of chromosome No. 22 onto the distal end of the long arm of chromosome No. 9, i.e., t(9;22) (q34;q11). The Ph1-positive modal cells of the NALM-1 line also had two common marker chromosomes, an extra X-chromosome, and missing chromosomes in groups No. 7, 9, and 15. Immunologic examination of the NALM-1 cells revealed them to have non-T-non-B (null) surface characteristics. An antigen specific for cells of acute leukemia and a human la-like antigen were detected. These facts suggested that the NALM-1 cell line originated from CML cells and maintained the cytogenetic and Immunologic characteristics of such cells.