Heparinized blood from 43 patients with multiple sclerosis (MS) and 43 controls matched for age, sex and race were incubated at 42 degrees C for 24 h. The hematocrit values were determined before and after heating. The plasma and red blood cells (RBC) were separated and the plasma free fatty acids (FFA), total neutral lipids (NL; which included the triglycerides and cholesteryl esters), phospholipids (PL), and RBC lipids from 24 patients and 24 controls were isolated. The total amounts of the various FFA in the plasma and the relative percentage fatty acid compositions of the various FFA, NL, ester-linked fatty acids of plasma PL, RBC lipids as well as those of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) fractions of RBC lipids were determined by gas-liquid chromatography (GLC). The hematocrit values were significantly higher in heated samples from both MS and controls. The results of fatty acid analyses from 24 pairs of heated and unheated MS and controls showed that the concentrations of total FFA and most of the fatty acids increased significantly in the heated plasma of both MS and control samples, compared to the respective unheated plasma samples. When the FFA values were expressed as relative percentage compositions, the results showed significant increases in palmitic, arachidonic and adrenic acids and significant decreases in oleic, linoleic and linolenic acids in the heated samples from MS and controls compared to the corresponding unheated samples. In NL the relative percentages of linoleic and arachidonic acids were significantly higher in the heated samples from both MS and controls, while in the PL, the percentages of linoleic acid were lower in the heated samples from both MS and controls, when compared to the corresponding unheated samples. The fatty acid compositions of the RBC lipids showed highly significant increases in palmitic plus palmitoleic, stearic and oleic acids and significant decreases in linoleic and most of the polyunsaturated acids in the heated MS samples compared to the unheated MS samples. In contrast, the fatty acid compositions of the RBC lipids from heated controls showed significant increase in only palmitic plus palmitoleic acids and significant decreases in linoleic and arachidonic acids, compared to the values from unheated controls. The results show that the RBC membranes from MS were more affected by the heat than those of control RBC, indicating possible differences between blood from MS patients and controls.