The effect of a series of phytosterols on lipid chain ordering in 1-palmitoyl(H-2(31))-2-oleoyl-sn-glycero-3-phosphocholine (POPC-d(31)) multibilayer vesicles was examined by H-2 NMR spectroscopy at 25 degrees C. These results, along with existing data for other sterols, indicate that the ordering power of sterols in POPC-d(31) depends on subtle aspects of sterol structure. Cholesterol, 7-dehydrocholesterol (7-DHC), campesterol, beta-sitosterol, ergosterol, brassicasterol, and stigmasterol all increase the lipid chain order as sterol concentration is increased. However, saturation of the ordering occurs at different sterol concentrations for ergosterol (as previously reported), brassicasterol, beta-sitosterol, and stigmasterol. Here our interest lies in finding which part of the sterol structure is responsible for the observed saturation of the palmitoyl chain order as a function of sterol concentration. In particular, we propose that the saturation of the ordering of POPC-d(31)/brassicasterol and POPC-d(31)/stigmasterol membranes at quite low sterol concentrations is due to the presence of a double bond at C22. We also discuss how the structural differences between the sterols affect their ability to intercalate between the POPC acyl chains. Furthermore, the effective solubility of sterols in POPC is discussed in relation to the dependence of maximum POPC-d(31) chain order vs sterol concentration