Cheng (1958) reported that arboviruses of group B but not of group A were sensitive to proteases. Nicoli & Maydat (1965) apparently confirmed this although other reports suggested that protease sensitivity did not distinguish the groups. Merrill (1936) showed that equine encephalomyelitis virus (group A) was inactivated by chymotrypsin but not by trypsin. Takehara & Hotta (1961) found that Western equine encephalitis virus (group A) as well as the group B viruses of Japanese B encephalitis, dengue and yellow fever were sensitive to trypsin. Following the speculation by Bachrach (1963) on the taxonomic significance of the test, we decided to test 33 representative strains of arboviruses against trypsin, chymotrypsin and pronase. Trypsin (Bovine type XI DCC treated No. T 1005) and α-chymotrypsin (Bovine pancreas type II No. C-4129) were purchased from the Sigma Chemical Company (St Louis, U.S.A.). Pronase (B Grade No. 53703) was purchased from Calbiochem (Los Angeles, U.S.A.).