Abstract: Compounds comprising polypeptide derivatives of the formula ##STR1## wherein R.sub.1 is the amino acid sequence of the polypeptide minus the terminal amino acid; R is the side chain group determining the identity of the terminal amino acid; and Z is ##STR2## or --CH.sub.2 X wherein X is Cl, Br, or I; are provided. Precursor peptides are peptide hypothalamic releasing and inhibitory factors, opiate peptides, and fragments thereof. The derivatives act as antagonists or agonists to the precursor peptides and are particularly useful in bioassay procedures.