Abstract
Since the extensive use of cynomolgus monkey kidney cell cultures (MK) for virus researches, contaminations of MK with simian viruses (SV) have been serious problems in virus laboratories (Endere and Peebles, 1954; Rustigian et al., 1955; Hull et al., 1656; Hull and Minner, 1957; Hull et al., 1958) . In the National Institute of Health of Japan (NIH), about ten monkeys have been used every week since 1961 and every batch of MK has been checked for the contamination of SV since 1962. The following is a part of the results of SV examination (from January 1962 to March 1962), which showed a high rate of contamination by hemagglutinating SV 5. The communication also contains some analysis about the origin of the contamination.
