Cytolytic activity of Met-enkephalin, an endogenous opioid peptide, was studied within the 10(-7)-10(-17) M concentration range in K562 human erythroid leukemia cells. Cytolytic activity was determined by the trypan blue inclusion method after 13, 15 and 18 h of Met-enkephalin co-incubation with target cells. Discrete maxima of cytolytic activity were detected at concentrations of 10(-9)-10(-10), 10(-13) and 10(-15) M. Cytolysis was accompanied by internucleosomal DNA fragmentation which is indicative of the mechanism of cell death being apoptosis.