ABSTRACTIn vitroinhibition of marrow granulopoiesis was produced by a well-characterized human neuroblastoma cell line (CHP 134). A standard double layer, semi-solid agar system was employed in the experiments. The inhibition was present whether the neuroblastoma cells were mixed with the marrow cells or whether they were separated in a contiguous agar layer. Irradiation of the neuroblastoma cells lessened the inhibitory effect but did not eradicate it. Medium conditioned by the neuroblastoma cells had a mild, but not statistically significant, suppressive effect upon granulopoiesis. Additional studies to define the precise mechanism of suppression are under-way.