A transplantable granulocytic leukemia (GL-13) in inbred strain 13 guinea pigs had been shown previously to have many characteristics in common with human CML except that a blast crisis did not occur in the terminal stage of the disease. We report here that, after the 25th transplant generation, leukemic guinea pigs began to develop a blast crisis similar to that seen in human CML. This feature of the disease is still present after 90 transplant generations. The transplantation conditions necessary to produce the leukemia (now referred to as GL-13-BC) with a predictable clinical and hematological course over a short interval are described. Thus, s.c. injection of 3.0 X 10(6) leukemic cells results in a slowly rising leukocyte count, due mainly to increasing numbers of myelocytes and mature granulocytes, at about 3 weeks after injection. At this early stage the spleen weights of the leukemic animals are about seven times greater than those of normal controls. Approximately 4-7 days later the leukocyte count rises sharply due to a blast crisis and the animals die shortly thereafter with a mean survival time of 30 (range 25-32) days. It is proposed that the GL-13-BC leukemia could serve as a useful model for investigating the problem of blast crisis associated with human CML.