The technique of gas-liquid chromatography-chemical ionization mass spectrometry can easily identify trace peaks and unresolved peaks on gas-liquid chromatography, utilizing MH(+) ions of chemical ionization mass spectra. In polyunsaturated fatty acid methyl esters such as C22:5 and C22:6, the determination of molecular weights that are difficult to determine by electron impact mass spectrometry could be easily identified by chemical ionization mass spectrometry. The identification could be performed even more easily from a mass chromatogram obtained by means of a gas-liquid chromatography-chemical ionization mass spectrometry-computer system. This technique was applied to analysis of fatty acid methyl esters of triglycerides, cholesteryl esters, nonesterified fatty acids, and phospholipids obtained from normal human serum. It was observed that odd-carbon-number fatty acids contained more isomers of different types than even-carbon-number fatty acids, whether the fatty acids were saturated or unsaturated. As for the types of isomers, we presume the existence of iso, anteiso, branched, cyclic fatty acids, and, in the case of unsaturated fatty acids, positional isomers. The qualitative determination of the group of fatty acid isomers containing 20 carbon atoms was also investigated as well as the application of procedures for quantitative determination. Sensitivities for saturated fatty acids differ from those for unsaturated fatty acids of the same carbon number, and sensitivities for unsaturated fatty acids of the same carbon number differ with the number of double bonds because the amounts of MH(+) - 32 and MH(+) - 32 - 18 fragment ions formed are different.