Gelfoam sponge cultures of stromal cells, derived from guinea pig marrow-depleted bone fragments, support the growth of myeloid precursors in the absence of exogenous stimulants. EM radioautographic studies were undertaken to identify active synthesis and distribution of 3H-fucose-containing glycoproteins within the 3-D organization of cultured hemopoietic tissue. Labeling of 5-14 day cultures with tritiated fucose for 20 min revealed an intracellular localization of silver grains over the endoplasmic reticulum and Golgi bodies of all stromal cells. Cumulative labeling (1-48 h) of similar cultures indicated that (i) silver grains subsequently accumulated in plasma membranes, large cytoplasmic granules and also within lipid droplets, (ii) primitive and myeloid cells were also labeled. Localized extracellular concentrations of silver grains were facilitated by the multiple inter-stromal cell contacts and by the accumulation of fine labeled cytoplasmic processes. Mechanisms whereby stromal cells may actively influence physiochemical changes in the hemopoietic inductive microenvironment are suggested.