Immunophenotypes and genotypes were analyzed for a case of chronic myelocytic leukemia in blast crisis (BC). In the early stage of BC (early BC), the blasts consisted of lymphoid-myeloid cells, but in the later stage of BC (late BC), they were myeloid cells, morphologically and phenotypically. On Southern blot of DNAs in early BC, a single rearranged fragment of immunoglobulin heavy chain (IgH) genes was detected, whereas IgH genes were in germline configuration in both initial chronic phase and late BC. A clone which had a rearranged IgH gene in early BC, was considered to have co-existed with a clone which had the germline IgH gene. Analysis for bcr genes confirmed that the chronic phase as well as early and late BC were of the same clonal origin. Phenotypic and immunogenotypic analyses, however, revealed that at least two secondary clones emerged from the primary clone. The therapeutic effect against lymphoid population among mixed crisis cells could be evuluated not only phenotypically but also genotypically.