Phosphoglucomutase 1, an enzyme mapping on the short arms of chromosome 1, is constantly missing in the leukemic cell line K-562 in spite of the presence of three No. 1 chromosomes. In the present work, a subclone of the cell line, K-562 (S)P, is described, where the enzyme can be demonstrated, thus excluding a small deletion as the cause for the lack of expression of phosphoglucomutase 1. The relationship between the presence of the enzyme and the karyotype changes in this subclone is analyzed. Addition of several inducers to the standard K-562 line failed to elicit expression of the enzyme.