During the course of the survey of nonprotein amino acids, especially cysteine derivatives in the plants belonging to the family Liliaceae, a new amide, isoseryl S-methylcysteamine sulfoxide, was isolated from the tubers of Sarutori-Ibara (Smilax china, Liliaceae). Two components obtained by hydrolysis of the amide were identified as isoserine and S-methylcysteamine sulfoxide, respectively, by FD-high MS spectrum and comparison with authentic samples. This paper deals with the isolation and structural determination of the new amide.