Epidemiologic studies and human volunteer experiments performed during and after World War II showed that hepatitis (hepatitis A) was caused by a filterable agent that survived ether extraction. Although infectious hepatitis was distinguished from homologous serum hepatitis (hepatitis B) in the 1940s, the specific etiologic agent resisted identification for another three decades. Classic virological methods were first applied to the isolation and identification of the etiological agent; these included small animal inoculation, inoculation of embryonated chicken eggs, and inoculation of in vitro cell cultures. During the expansion of cell culture techniques in the later 1940s and 1950s, many of the most important human viruses were isolated and identified. The most important of these was undertaken by W. A. Rightsel, J. W McLean, and coworkers at the Parke-Davis Company. The Detroit-6 system was also established in Australia by Cole, a member of the original Parke-Davis team.