T antigen (neoantigen) of Simian Virus 40 has been localized at the ultrastructural level in infected MA 104 cells in culture. The cells were fixed 15 min. to 1 hr in 1% formaldehyde, freshly prepared from paraformaldehyde, in 0.2 m cacodylate buffer, pH 7.3, + 0.25 m sucrose. This allowed adequate conservation of ultrastructure while permitting subsequent penetration of reagents into the cells. The antigen was revealed by successive exposure to unlabelled hamster serum containing antibody against the T antigen and to labelled rabbit antibody against hamster immunoglobulin. The latter conjugate of rabbit antibody was labelled with peroxidase for which the cytochemical reaction results in an osmiophilic, electron dense reaction product which can be visualized by electron microscopy. The T antigen of SV40 first appeared in a reticular network throughout the nucleus, except the nucleolus, in a position which seems to correspond with that of the chromatin network.