CD1 monoclonal antibodies were assayed on peripheral blood mononuclear leukocytes (PBML) of type 1 human immunodeficiency virus (HIV1)-infected patients using immunogold technique. Using IOT6 monoclonal antibody, a significant increase of the CD1 positive cells per microliter of blood was found in acquired immunodeficiency syndrome (AIDS) and AIDS-related complex (ARC) patients (265 +/- 34/microliters, n = 44 and 491 +/- 64/microliters, n = 36, respectively) as compared to controls (108 +/- 11/microliters, n = 43, P less than 0.001). These findings were confirmed with four other CD1 monoclonal antibodies in six patients. Characterisation of these CD1 positive cells showed that they were double stained with either CD4 or CD8 monoclonal antibodies. Moreover, cytochemical analysis of these cells showed the absence of myeloperoxidase activity and ultrastructural examination did not reveal Birbeck granules, well known to characterise the Langerhans cells. Further investigations are warranted to assess the biological and clinical relevance of these findings.