The cellular metabolism of albumin- and lipoprotein-bound 18:2(n - 6) during monocytic differentiation was examined in the human premonocytic U937 and Mono Mac 6 cells. Differentiation for 72 h of U937 cells with retinoic acid (RA, 1 microM) or 1,25-(OH)2-vitamin D3 (1,25-D3, 10 nM) and of Mono Mac 6 cells with RA (1 microM) or lipopolysaccharide (LPS, 10 ng/ml) increased the desaturation and elongation of [1-14C]18:2(n - 6) to [1-14C]20:4(n - 6). In undifferentiated U937 and Mono Mac 6 cells, incubations with human LDL (100 micrograms/ml, 18 h) resulted in a 2.5-fold increase in 18:2(n - 6) levels in the cellular phospholipids. Differentiation of U937 cells with RA or or of Mono Mac 6 cells with LPS prior to LDL addition. Significantly reduced 18:2(n - 6) and elevated 20:4(n - 6) levels in cellular phospholipids. This increase in 20:4(n - 6) was likely not due to an increased incorporation of preformed 20:4(n - 6) esterified in LDL, as the receptor-specific degradation of [125I]LDL was reduced in both the RA-treated U937 and LPS-treated Mono Mac 6 cells. In U937 cells incubated with [1-14C]18:2(n - 6), the synthesis of TXB2, PGE2 and HHT could be detected after differentiation with RA. suggesting the availability of [1-14C]20:4(n - 6), derived from [1-14C]18:2(n - 6), for cyclooxygenase metabolism. Our results show that the conversion of 18:2(n - 6) to 20:4(n - 6) increases during monocyte differentiation. The 18:2(n - 6) supplied to the cells via the receptor-mediated uptake of LDL was utilized as substrate for the increased 20:4(n - 6) synthesis.