Indirect immunofluorescence was used to show the presence of galactocerebroside (galC), a lipid found in myelin, on the surface of about half of the Schwann cells isolated from neonatal rat sciatic nerves and cultured for 1 day without neurons. By day 4 in vitro, the Schwann cells had all lost their surface galC. Three days after beginning treatment with 10(-3) molar 8-bromo-adenosine 3',5'-monophosphate (8-bromo cyclic AMP) or N6,O2'-dibutyryl adenosine 3',5'-monophosphate (dibutyryl cyclic AMP), galC reappeared on the Schwann cells, and 2 days later 48 percent of the cultured Schwann cells showed surface galC. Tritium from tritiated D-galactose was incorporated into galC by the 8-bromo cyclic AMP-and dibutyryl cyclic AMP-treated cultures at a rate 15 times the control rate.