The addition of 1% (w/v) carboxymethyl cellulose to the culture medium induces the formation of neurites of clone N18 neuroblastoma cells even in the presence of normal (5–10%) serum supplement concentrations, which rivals that previously observed by growth in low 0.1% serum. Heavy metal ions associated with the carboxymethyl cellulose were responsible for small increases in the sizes of cell bodies during treatment. Pretreatment of the PC 12 pheochromocytoma line of neuroblasts with carboxymethyl cellulose for 1 day prior to their stimulation with nerve growth factor resulted in an acceleration in the rate, but not extent, of neurite outgrowth.