The aim of this work was to study the fatty acid metabolism of the human-hepatoma cell line Hep G2. The cultured cells were incubated with either a saturated (palmitic, stearic) or a polyunsaturated (linoleic, alpha-linolenic, eicosatrienoic n-6) radioactive fatty acid. The fatty acids were incorporated into all the basic lipid classes as well as into the main phospholipid subclasses in the cellular membranes. All the fatty acids tested provided a source of carbon for lower members of the saturated fatty-acid family or for cholesterol through beta-oxidation and a new cycle of de novo synthesis. Moreover, all radioactive fatty-acid precursors, whether saturated or unsaturated, were anabolized to higher derivatives within their own family. In the case of saturated fatty acids, palmitic and stearic, they were readily monodesaturated to their corresponding products, thus demonstrating the presence of a delta 9 desaturase. Linoleate and alpha-linolenate were both desaturated and elongated to all the subsequent members of their respective n-6 and n-3 families. These latter observations provide evidence for the incidence of desaturation at the 6 and 5 positions along with the existence of an elongating capacity for fatty acids of all families and chain lengths. In addition, the cellular steady-state fatty-acid profile was seen to be significantly different from the spectrum of exogenous fatty acids available in the growth medium. We conclude that the Hep G2 human-hepatoma line represents an appropriate and relevant experimental model system for investigating the fatty-acid metabolism of adult human liver in vivo.