The toxic protein ricin (also known as RCA60), found in the seed of the castor plant (Ricinus communis) is frequently encountered in law enforcement investigations. The ability to detect ricin by analyzing its proteolytic (tryptic) peptides by liquid chromatography-tandem mass spectrometry (LC-MS/MS) is well established. However, ricin is just one member of a family of proteins in R. communis with closely related amino acid sequences, including R. communis agglutinin I (RCA120) and other ricin-like proteins (RLPs). Inferring the presence of ricin from its constituent peptides requires an understanding of the specificity, or uniqueness to ricin, of each peptide. Here we describe the set of ricin-derived tryptic peptides that can serve to uniquely identify ricin in distinction to closely-related RLPs and to proteins from other species. Other ricin-derived peptide sequences occur only in the castor plant, and still others are shared with unrelated species. We also characterized the occurrence and relative abundance of ricin and related proteins in an assortment of forensically relevant crude castor seed preparations. We find that whereas ricin and RCA120 are abundant in castor seed extracts, other RLPs are not represented by abundant unique peptides. Therefore, the detection of peptides shared between ricin and RLPs (other than RCA120) in crude castor seed extracts most likely reflects the presence of ricin in the sample.