The Id family genes are differentially expressed during the early stages of normal hematopoietic development. Murine bone marrow was plated in methylcellulose at low density in the presence of IL-1, IL-3, SCF, and erythropoietin, then allowed to grow until individual colonies contained four to eight cells. At that time, colonies were separated and individual cells removed for single-cell RT-PCR analysis of their gene expression. Five micrograms of the amplified product from 40 such progenitor cells was Southern blotted and probed with high specific activity probes for each of the Id family genes and for GAPDH. Phenotype was assigned by May-Grunwald-Giemsa staining of the mature cells that developed from each colony start. E, erythroid; Meg, megakaryocytic; Mac, macrophage; Neut, neutrophil.