We treated KU812 cells from different passage levels with compounds which induce differentiation of other leukemic cell lines. Early passages contained small subpopulations of basophil and erythrocyte-like cells, but later passages consisted of more uniform cells with a myelomonocytic phenotype. When cultured with 1 nM actinomycin D, the cells became nonproliferative, developed pale cytoplasm and segmented nuclei, and lost nonspecific esterase activity. Thus, KU812 cells can be induced to mature to cells with a myelomonocytic phenotype. However, their expression of previously reported basophil characteristics diminishes with continued passage, and may be a capability of only a subpopulation of the cells.