In a 24-hr time course study we reported previously that a single systemic injection of morphine profoundly affected various immune parameters in mice. In the present study we examined whether these effects are mediated by changes in opioid receptor density on murine splenocytes after acute in vivo morphine (20 mg/kg s.c.) and methadone (12.5 mg/kg s.c.) at equianalgesic doses. To define the splenocyte subpopulations we used flow cytofluorimetric analysis with specific fluorescent monoclonal antibodies and calculated the binding of the fluoresceinyl opiate antagonist naloxone on opiate receptors. Both morphine and methadone reduced the density of opiate receptors on B- and T-lymphocytes. Specifically, 20 min, 1 and 3 days after the injection there was a marked reduction (about 55%) in naloxone binding sites; these returned to base line after 5 days for T-lymphocytes and after 7 days for B-lymphocytes. Despite the low proportion of macrophages among total splenocytes (about 10%), our results also indicate a tendency to a reduction in opiate receptor density also in the macrophage population. These findings indicate that a single exposure to morphine and methadone results in a strong, lasting down-regulation of opiate binding sites in murine splenocytes, probably accounting for the immunomodulation induced by opiates.