RB 101 [HzNCH (CHzCH~ CH~ CH~ SCHzCH (CHzPh) CONHCH (CH~ Ph) COOCHzPh] after iv administration were three times greater than those of a similar combined dose of its two constitutive moieties. The separation of the two diastereoisomers constituting RB 101 showed that the analgesia has a stereochemical dependence, the (S, S, S)-isomer being more active than the (SJZ, S)-isomer. Furthermore, in the tail flick test in the rat, RB 101 ...