The locations of human T-cell leukemia (HTLV-I) antigens in HTLV-I and HTLV-Iproducing T-cells were studied with special reference to the inner structure proteins (MA, p19 and CA, p24) by electron microscopy using the peroxidase-labeled antibody method with monoclonal antibodies. ATL-associated antigens (ATLA) were detected by electron microscopy of cells incubated in cell suspension with sera from adult T-cell leukemia (ATL) patients on the surface of virus particles and the plasma membrane, but not inside the virus and cells. Intraviral and intracellular antigens were detected by the immunoelectron microscopy of frozen-sectioned specimens of paraformaldehyde-fixed cells. The viral matrix protein (MA, p19) was detected with the anti-p19 monoclonal antibody in the viral envelope, the viral matrix and some parts of the plasma membrane. The viral capsid protein (CA, p24) was detected with the anti-p24 monoclonal antibody in the viral core. The positive reactions detected on the viral envelope, plasma membrane, endoplastic reticulum, and nuclear envelope with sera of ATL patients were deduced to be mainly due to the viral envelope proteins and their precursors or to proteins fused to the viral envelope proteins.