The electron impact mass spectra of (2R, 4R)-2-(2-hydroxyphenyl)-3-(3-mercaptopropionyl)-4-thiazolidinecarboxylic acid (1), SA 446, and the related compounds 2-7 are discussed. The fragmentation patterns were classified into three types of characteristic ring fragmentation (1, 3-, 2, 5- and 3, 5-cleavages) with or without substituent cleavage. The fragment ions were established by the shifting method with the labelled derivatives 4'and 4"and the elemental compositions were confirmed by high-resolution mass spectrometry.