An immunohistochemical investigation of postmortem lividity was performed to illuminate localization of hemoglobin (Hb) and the mechanism of fixed lividity. The fixed lividity was defined as an unfading phenomenon by thumb finger pressure. Skin specimens were taken from 68 autopsy cases 7.5-336 h (2 weeks) postmortem. Localization of Hb of the specimens was examined by a labeled streptabidin biotin (LSAB) method using polyclonal (rabbit antihuman hemoglobin antibody) and monoclonal (mouse anti-human hemoglobin monoclonal antibody) antibodies. Positive staining for Hb was observed in various sites of the skin, i.e. in only intravascular erythrocytes, in vascular walls and perivascular tissue including sweat glands and sebaceous glands, in the dermal connective tissue, and in almost all of skin tissue except the horny layer. The diffusion of Hb into skin tissue was observed in 20 of 41 displaceable lividity cases (49%) and 11 of 27 fixed lividity cases (41%). Compared to displaceable lividity, superficial plexi in fixed lividity were filled with erythrocytes, which were markedly immunodetected. These findings support the hypothesis that the fixation of lividity is not due to diffusion of Hb into skin tissue but hemoconcentration in blood vessels.