During production, control cultures are generally established in parallel to the cultures used for the production of polio vaccine. Van Steenis et al. (1980) compared the frequency of contaminated primary, secondary, tertiary, and quaternary cultures (flasks as well as microcarrier cultures) and stated that all cultures derived from captive-bred animals were free of virusinduced cytopathic effects or hemadsorption, whereas 30 out of 45 kidney cultures from single imported wild caught animals and all of 71 cultures derived from multiple animals showed cytopathology (Table II) .