Immunoelectron microscopic observations of human immunodeficiency virus (HIV) antigens were made using the indirect peroxidase-labeled antibody method with monoclonal antibodies (MoAb) to the envelope glycoprotein (gp160/120), matrix protein (MA, p17), and capsid protein (CA, p24). Antigens in the virions and HIV-infected cells were detected in frozen-sectioned specimens. Positive immunoreaction of MoAb to gp160/120 was located on the viral envelope, plasma membrane, endoplasmic reticulum, and nuclear membrane. The reaction of MoAb to p17 was at the inner leaflet of the viral envelope and the nucleoid. The reaction of MoAb to p24 was localized on and around the viral core. In cell suspension, however, positive immunostaining was observed only on the surface of the viral envelope and plasma membrane with MoAb to gp160/120, and no positive staining was observed with MoAb to p17 and MoAb to p24. The reaction of HIV-infected cells with normal control serum and all of the reactions in uninfected cells with these MoAbs were negative. It appears that antibodies have difficulty penetrating into virions and cells in cell suspension.