Four types of colonies can be formed by progenitor cells (CFUc) in double layer agar cultures: eosinophil, granulocyte, macrophage, and mixed granulocyte-macrophage. We have developed a method for in situ staining of intact agar plates with luxol fast blue and acetoorcein employed to differentiate the type of colonies. Cultures of 10(5) normal human bone marrow cells yielded 89.1 colonies, of which 72% were eosinophil. Cultures of 5 X 10(5) peripheral blood cells, enriched by isopycnic sedimentation, yielded 39.0 colonies, of which 48% were eosinophil. T-cell depleted peripheral blood mononuclear cells yielded 116.9 colonies/5 X 10(5) cells, of which 74% were eosinophil. Umbilical cord blood cells yielded 65.1 colonies/5 X 10(5) cells, of which 23% were eosinophil. Comparing similar numbers of total colonies, there appears to be fewer relative and absolute numbers of eosinophil colonies formed by CFUc recovered from cord blood as opposed to adult peripheral blood and bone marrow. Enumeration of colony numbers without regard to types of colonies is imprecise. The method employed in the present study is easy to perform, reproducible and provides permanent slides.