1. The authors describe a case of acute leukemia in a man of 36 years. During the course of his illness, a change in the agglutinability of the red cells was observed. The cells, originally A1, changed continuously so that in some samples it was possible to detect as many as three populations of red cells, namely, A1, weak A, and so-called non-A. In the course of the illness, fluctuations in the numbers of these cells in the whole population could be observed.
2. The clinical course and the results of detailed isoserologic examinations are presented.
3. Possible causes of these changes are discussed: (a) Somatic mutation; (b) chromosome abnormalities; (c) metabolic changes of the hemopoietic tissue.