We have reported that melatonin exerts colony stimulating activity and rescues bone marrow cells from apoptosis induced either in vivo or in vitro by cancer chemotherapy compounds. We proposed that melatonin regulates interleukin-4 (IL-4) production in bone marrow T-helper cells and that IL-4 stimulates adherent stromal cells to produce colony stimulating factors (CSF). However, in further investigations we did not find any direct evidence of the ability of melatonin to stimulate IL4. We found that besides anti-IL4 monoclonal antibody (mAb), the opioid antagonist naltrexone also neutralized the colony stimulating activity and part of the hematopoietic protection exerted by melatonin. SDS-PAGE and immunoblotting analysis of supernatants of bone marrow T-helper cells incubated overnight with melatonin revealed the presence of two proteins with an apparent molecular weight of 15 and 67 kDa, which were recognized by both anti-common opioid sequence (Tyr-Gly-Gly-Phe) and anti-IL4 mAbs. When Abs against known opioid peptides were tested, only anti-dynorphin B Ab labeled the 67 kDa but not the 15 kDa protein. These melatonin-induced-opioids (MIO) were separated by gel filtration. The lower molecular weight MIO (MIO15) seems to mediate the naltrexone-sensitive hematopoietic effects of melatonin. Consistently, we found the presence of opioid receptors in adherent bone marrow cells. Apparently, the higher molecular weight protein, MIO67, was responsible for the naltrexone-insensitive part of the melatonin-induced hematopoietic rescue. These melatonin-induced T-helper cell products which resemble both IL-4 and dynorphin B might represent a new family of opioid peptides with hematopoietic and immune functions.