The response of human myeloid leukemia cells to various sources of colony-stimulating activity (CSA) and media conditioned by phytohemagglutinin-stimulated mononuclear cells (PHA-LCM) was investigated in liquid and colony culture. PHA-LCM, placenta-conditioned medium, GCT cell line-conditioned medium, leukocyte-conditioned medium, and partially purified CSA for human and murine cells were tested for ability to support growth of granulocyte-macrophage colonies from adherent cell-depleted human bone marrow. This activity was correlated with ability to support leukemia colony growth in methylcellulose, and [3H]thymidine incorporation in liquid culture by normal bone marrow cells, leukemia cells, and the KG-1 myeloid leukemia cell line. For normal cells, growth and liquid culture responses were highly correlated for various sources of CSA (r = 0.92), and addition of data using PHA-LCM changed results only slightly (r = 0.89). [3H]thymidine incorporation by leukemia cells from patients without a prior history of a myeloproliferative disorder was also highly correlated with normal CSA (r = 0.97) for sources other than PHA-LCM. Responses of leukemia blasts and KG-1 cells in liquid culture to PHA-LCM appeared in excess of its CSA for normal cells. Colony growth by leukemia cells was not clearly correlated with either liquid culture activity for leukemia cells or CSA for normal cells. PHA-LCM was also not statistically superior to placenta-conditioned medium as stimulus for leukemia colony growth, but was superior to placenta-conditioned medium for some patients. Differentiation in culture did not appear to depend on CSA source. We conclude that normal myeloid cells respond to CSA in a highly correlated fashion in both colony and liquid cultures. The majority of myeloid leukemia cells respond to either PHA-LCM or CSA, but the ability of PHA-LCM to support leukemia cell growth is greater than its CSA content. The possibility exists that overlapping populations responsive to CSA and to PHA-LCM are present simultaneously in patients with myeloid leukemia.