We have investigated the hydrolysis and protection from hydrolysis of several peptides by plasma enzymes and by the plasma components previously described as inhibitors of enkephalins' hydrolysis. The results shown indicate that all the peptides actually hydrolyzed are also partially protected from hydrolysis by the enkephalin-protecting substances. Protection is fairly uniform for all the peptides tested, but considerably higher in the case of leu- and met-enkephalin, suggesting a partial specificity of the protecting substances towards opioid peptides.