In recent years interest has been aroused in the use of fish oils rich in polyunsaturated fatty acids for protection against and treatment of a number of disorders including artherosclerosis (Kramhout et al., 1985; Bradlow, 1986; Leaf & Weber, 1988), inflammation (Goetzl et al., 1986; Bittmer et NI., 1988) and autoimmune diseases (Robinson et al., 1986). Monocytes and macrophages are involved in the development of artherosclerotic plaques (Mitchinson & Ball, 1987) and in inflammatory and immune responses (Werb & Goldstein, 1987). While monocytes/macrophages are not the 'target' of fish oil treatment, it is reasonable to assume that if the dietary polyunsaturated fatty acids lead to alteration of plasma lipid profiles, as is proposed (Phillipson et al., 1985), then the lipid content and composition of macrophages may be affected. Macrophage functions such as secretion, phagocytosis and interaction with other cell types involve intracellular and/or plasma membranes. If macrophage membrane structure is altered cellular function may be affected. Therefore, it is of interest to determine the effect of altered fatty acid composition upon macrophage function. The fatty acid composition of a variety of cell types can be modified by exposing them to fatty acids complexed to bovine serum albumin (BSA) (Spector et af., 1981) and this approach has been used previously with macrophages (Mahoney et al., 1977, 1980; Lokesh & Wrann, 1984; Lokesh et d, 1988). However, these latter studies have not used a wide range of fatty acids; in particular, the effects of long-chain polyunsaturated fatty acids have not been invcstigated. Furthermore, the effect of fatty acid modification upon macrophage functions other than phagocytosis and pinocytosis (Mahoney et al., 1977, 1980; Schroit & Gallily, 1979; Lokesh & Wrann, 1984) has not been determined. Murine thioglycollate-elicited peritoneal macrophages wcre prepared and purified as previously described (Newsholme et al., 1986). The cells were cultured in minimum cssential medium supplemented with 5 mM-glucose and 2 mwglutamine at 37°C in an air/C02 (19:l) atmosphere at a density of 2 x 10' cells/plate. The cell culture medium was supplemented with 5% (v/v) fetal calf serum, 5% (v/v) mouse serum, 0.1/0 (w/v) BSA, or 0.1% (w/v) BSA-fatty acid complex (0.3 mM-fatty acid). After a period of 2-4 days, the cells were washed thoroughly and harvested. Total lipid was cxtracted using chloroform/methanol (2:1, v/v) as described by Folch et al. (1957). Neutral lipids were separated from phospholipids by chromatography through a column of activated silicic acid prewashed with chloroform. The neutral and polar lipids were eluted from the column with chloroform and chloroform/methanol (1: 1, v/v), respectively. Fatty acids were prepared by saponification with methanolic 0.5 M-KOH for 60 min at 70°C and were extracted into ethyl acetate. Fatty acid methyl esters were prepared by reaction with an excess of diazomethane, before separation and identification by g.c.-m.s. (Wing ef al., 1984). The fatty acid composition of the macrophage varies markedly according to the composition of the cell culture medium (Table 1). The BSA-grown cells (no fatty acid source present in the medium) are rich in palmitate (16:0), stearate ( 18:O) and the shorter chain saturated fatty acids, laurate