The health value for man of lipids in bovine muscles can be improved by the addition of PUFA to the animals' diets, but such treatments can modify fluidity of plasma lipoproteins and therefore their metabolic functions. The aim of the present study was to analyse whether changes in chemical composition of lipoproteins in steers fed sunflower oil-rich diets altered lipoprotein fluidity, measured by fluorescence polarization and electron spin resonance. LDL, light HDL and heavy HDL fractions were isolated by ultracentrifugation from plasma of eighteen crossbred Charolais x Salers steers. For a period of 70 d, animals were given a control diet (C, n 6) consisting of hay (540 g/kg) and concentrate mixture (460 g/kg) or the same basal diet supplemented with sunflower oil rich in n-6 PUFA (40 g/kg diet DM), given either as crushed seeds (S, n 6) or as a free oil infused directly into the duodenum (O, n 6), thus avoiding ruminal hydrogenation of PUFA. We have shown that in bovine animals: (1) fluidity measurements by fluorescence polarization must be made at the bovine physiological temperature (38.5 degrees C); (2) heavy HDL always appear as the less fluid lipoparticles; (3) electron spin resonance, which does not depend on lipoparticle size, is more appropriate to compare the fluidity of LDL with that of light HDL. The values for lipoprotein fluidity measured by both methods indicated that linoleate-rich diets did not have any effect when compared with diet C; however, chemical variables support a fluidification of lipoparticles, since in steers given the diet O, n-6 PUFA concentrations increased in polar (x1.8) and neutral (x1.6) lipids in lipoparticles (P=0.0001). The phospholipid:protein ratio increased in light (+20 %, P=0.019) and heavy (+23 %, P=0.06) HDL and especially in LDL (+46 %, P=0.0001); the total cholesterol:phospholipid ratio decreased in the three lipoprotein classes (-15 to -30 %, NS). Diet S led to similar but less pronounced effects. We concluded that linoleate-rich diets modified the chemical composition of plasma lipoproteins in steers, but did not alter their fluidity; this probably occurred as a result of 'homeoviscous adaptation', which ensured their functional capacity.