Monoclonal antibodies against the Leucocyte Common Antigens LC and MAC-387 expressed by lymphocytes, mononuclear phagocytes and polymorphonuclears failed to mark the cells of the pulmonary interstitial connective tissue in cases of SIDS. Controls with interstitial pneumonia showed clearly marked inflammatory cells. Although focal proliferation of cells was frequently observed in the vicinity of distended lymphatics in SIDS cases, an inflammatory origin as in the cases of interstitial pneumonia could not be confirmed. A relationship between interstitial edema and a non-inflammatory proliferation of the cells is discussed.