The beta -casomorphin-5 sequence was systematically modified by substitution of the naturally occurring amino acids. The derivatives are compared on the basis of their affinities towards mu 1-, mu 2 and delta -opioid binding sites estimated by means of binding studies with [3H]dihydromorphine and [3H]D-Ala2-Leu5-enkephalin as labels and computer curve fitting. A C-terminal amide group which is known to increase mu-site affinity of beta -CM-4 and beta -CM-5 mainly enhances the mu 2-site affinity. Furthermore, the Pro4-amide structure, which seems to be responsible for the affinity enhancement can be substituted by the pyrrolidide ring structure. Modifications in position 2, 3 and 4 can lead to an increase in mu 1-, mu 2- or delta -site affinity and/or selectivity. The specificity of these effects might be dependent on the respective changes in the charge or hydrophobicity due to the introduction of other amino acids. The results suggest firstly that the alternating proline residues in the beta -CM-5 molecule are not absolutely necessary for its mu-site affinities, and secondly that both opioid receptor subtype affinity and selectivity may be modified by changing charge and/or hydrophobicity in the middle part of the beta -casomorphin molecule.