Hemopoietic cells that adhered to preformed selected marrow stromal cell layers were characterized on the basis of progenitor cell production capacity, colony-formation kinetics and forward light scatter (FLS) properties. It was shown that early hemopoietic cells attached to the stromal layers within 2 hours of incubation, and were responsible for the initial production of the more differentiated granulocyte-macrophage colony-forming cells (day 14 GM-CFC) in long-term cultures (LTC). In a clonogenic assay system, hemopoietic cells that adhere to stromal layers can be detected by the formation of small colonies of blast-like cells and are designated as stroma-adherent CFC. Cell fractionation on the basis of FLS and counting colonies on days 5, 14 and 21 revealed that there was a succession of colony formation, indicating that the stroma-adherent CFC consisted of subpopulations with different lag-phases before initiation of proliferation. Day 5, day 14 and day 21 stroma-adherent CFC were shown to have a high, intermediate and low FLS, respectively. The cells that produced GM-CFC by day 21 showed FLS properties similar to those of day 21 stroma-adherent CFC, suggesting a correlation between day 21 stroma-adherent CFC and CFC-producing cells in LTC. The CFC present on day 21 required the synergistic action of GM-CSF+IL-3 + stem cell factor (SCF) for optimal proliferation. The prolonged lag-phase, the low FLS and the multifactor-responsive progeny are properties similar to those reported for other early cells, and it is proposed that day 21 stroma-adherent CFC represent an early hemopoietic cell type whereas day 5 stroma-adherent CFC represent a more mature stage of differentiation.