The size of the hepatitis delta virus was determined by filtration of infectious plasma through polycarbonate membranes and the inoculation of filtrates into chimpanzees. Chimpanzees inoculated with filtrates of 50 nm and 30 nm, but not 15 nm filters, developed delta hepatitis. The minimum size of infectious hepatitis delta virus was estimated to be approximately 30 nm, which is consistent with measurements of particles thought to be the virus.