Attempts to adapt six diploid cells to grow in monolayer cultures in serum-free media during the past 6 months have met with mixed success. The best results obtained thus far were with rhesus monkey kidney cells MK-2 which have been passed serially six times in serum-free Waymouth media containing 5 microgram sodium oleate and 2 mg of fatty acid-free bovine serum albuminm1 WO5. Human foreskin cells prepuce-50 have been serially passed two times in serum-free WO20 medium after being grown in WO20 with 2.5 newborn calf serum 11 serial passages. Neither of these two cell types have been serially passed sufficient times to know that they will remain stable. A series of eight media designed in our laboratory, containing different combinations of components found in fetal calf serum or newborn calf serum, were tested to grow African green monkey fetal lung cells DBS-FRhL-2 in serum-free medium.