Sakiyama et al. ('72) reported the isolation of a line of hamster cells (NIL 1c1) which contains only three glycolipids, hematoside, ceramide monohexoside and ceramide dihexoside. The incorporation of radiolabeled palmitate into hematoside during 24 hours was three fold higher in normal confluent, non growing cells than sparse, growing ones. Polyoma transformed cells did not exhibit this effect. We have continued studies with the untransformed cell line and have found that the higher incorporation of radiolabeled palmitate into hematoside by normal confluent cells is not due to a higher rate of turnover of hematoside at confluence but represents a true chemical increase. We have also found that this increase is not a gradual process during cell growth but instead occurs only when the cells become confluent and stop growing. The increase of hematoside at confluence is not due to a higher rate of synthesis of hematoside during G1, relative to the other phases of the cell cycle. We found the rate of synthesis of hematoside to be constant throughout the cell cycle. The rate of synthesis of phosphatidyl choline, phosphatidyl ethanolamine, phosphatidyl serine, phosphatidyl inositol and sphingomyelin was also studied as a function of the cell cycle. We found no large differences in the synthetic rate of any given phospholipid species throughout the cell cycle although the rate of synthesis of the glycerophospholipids was somewhat higher during late G1 and S. We did, however, find major differences in the rates of synthesis of the different phospholipid species.