Human parotid glands from 55 forensic autopsy subjects, 1-12 mo of age, were examined by immunohistochemistry without knowledge about the cause of death. Various combinations of monoclonal or polyclonal antibody reagents of the following specificities were applied in two-color immunofluorescence analyses: HLA class I or II (DR, DP, or DQ); pan-T cell (CD3); leukocyte common antigen (CD45); and secretory component (poly-Ig receptor). Sudden infant death syndrome victims (n = 17) were shown to have significantly increased numbers of CD45+ stromal leukocytes and intensified epithelial expression of HLA-DR and secretory component as well as increased endothelial expression of both HLA class I and II (DR, DP, and DQ) determinants compared with controls (n = 31) who had died from noninfectious causes. Seven overtly infectious subjects (bronchopneumonia) showed still more up-regulated expression. This result suggested that enhanced stimulation of the local immune system exists in sudden infant death syndrome, with release of certain cytokines that are known to up-regulate epithelial expression of HLA-DR and secretory component.