Refers to a compound (e.g., a protein) or a disease (e.g., a viral infection) or some other effect that only acts in/on one specific species of organism. For example, the antibiotic penicillin kills bacteria by blocking an enzyme which is critical for growth and repair of the bacterial cell wall (i.e., peptidoglycan layer), but penicillin does not harm other species (e.g., man). For instance, bovine somatotropin is a protein hormone that increases growth rate of young cattle and also increases the efficiency of mature cows in converting their feed into milk. Bovine somatotropin has no effect in humans, and (if eaten) is simply digested like any other food protein. It appears that most growth hormones are species specific.