The large T antigen of simian virus 40 (SV40) is a multifunctional regulatory protein, responsible for both the control of viral infection and the required alterations of cellular processes. T antigen is the only viral protein required for viral DNA replication. It binds specifically to the viral origin and as a helicase unwinds the SV40 DNA bidirectionally. The functional complex is a double hexameric oligomer. In the absence of DNA, but in the presence of ATP or a non-hydrolyzable analog, T antigen assembles into hexamers, which are active as a helicase when a partially single-stranded (3') entry site exists on the substrate. We have used negative staining electron microscopy, single particle image processing and three-dimensional reconstruction with a new algebraic reconstruction techniques (ART) algorithm to study the structure of these hexameric particles in the presence of different nucleotide cofactors (ATP, ADP, and the non-hydrolyzable analogs ATPgammaS and AMP-PNP). In every case a strong 6-fold structure was found, with the six density maxima arranged in a ring-like particle around a channel, and a well-defined vorticity. Because these structural features have recently been found in other prokaryotic helicases, they seem to be strongly related to the activity of the protein, which suggests a general functional model conserved through evolution.