The human monocytic leukemia cell line THP-1 is depleted of the long-chain n-6, AA, when compared to human monocytes. This reflects the low availability of this FA in the growth medium generally used for cultured cells. The effects of AA, as well as EPA, supplementation of THP-1 cells on the incorporation of these FA in cell PL, especially in PC and PE, was investigated. In addition the incorporation of labeled AA in PL from THP-1 cells was compared to that in human monocytes. Measurements were done through HPLC separation of PL, detected by UV absorption and radioactivity, FA analysis by GC and characterization of PC subclasses by FAB-MS. Marked differences were observed in the incorporation of the two FA in cell PL, particularly two PC subclasses, and in the accumulation in individual PL after supplementation of THP-1 cells. Accumulation of AA and EPA in THP-1 cells appeared to be mutually independent. The incorporation of AA was also quite different in THP-1 from that in monocytes. Thus, characterization of the FA content in lipids of cultured cells is an essential requirement for optimal utilization of these cells.