Human peripheral blood mononuclear cells (PBMC) obtained from healthy volunteers gave rise to three types of colonies that could be distinguished by their unique morphological characteristics; 5.1 +/- 0.9 (mean +/- SE) loose colonies of small cells, 2.1 +/- 0.9 packed colonies of larger cells, and 1.6 +/- 0.7 mixed colonies, were formed when PBMC (2 x 10(5) cells/dish) from 10 healthy volunteers were cultured in the medium containing methylcellulose. Cytochemical analysis with Astra blue-eosin dual staining revealed that all types of colonies consisted of various proportions of basophils, eosinophils, and hybrid (eosinophilic and basophilic) granulocytes which contained both of granules. These hybrid granulocytes were also identified by the ultrastructural features of two kinds of the granules. Relationships between cell numbers added to culture and formed colony numbers indicated colony of the cells to form the colonies. The colonies formed from untreated patients with chronic myelogenous leukemia (CML) during chronic phase were sevenfold of those from healthy volunteers. The colonies formed from treated patients with CML were normal in number. The number was 40 times greater in culture from a patient with basophilic crisis and a patient with myeloid crisis than normal, whereas that from a patient with lymphoid crisis were within normal limit. The number of the colonies from PBMC of patients with eosinophilia were in normal range, whereas those from bone marrow were six times or more than those from PBMC. These findings suggest that PBMC contains common basophil-eosinophil progenitors, and the culture used in this study in considered to be useful in the examination of basophil and eosinophil production from PBMC. Further studies using more purified cell population and other sources of colony stimulating factors such as interleukin-3 (IL-3), IL-4, IL-5, and granulocyte-macrophage colony stimulating factor should be carried out in order to clarify the significance of hybrid granulocytes in basophil and eosinophil proliferation and differentiation.