Summary Adenovirus stocks prepared by serial passage of the virus in rhesus monkey kidney cells were found to be contaminated with the simian papovavirus SV40. The infectivity titer and number of particles of SV40 in each of 4 virus stocks examined were higher than those of adenovirus. The virus stocks had an average adenovirus titer of 103.6 TCD50/ml and an adenovirus particle count of about 107.6 particles/ml. The same stocks had an average SV40 titer of 106.0 TCD50/ml and an SV40 particle count of about 109.0 particles/ml. Antigenicity of the adenovirus and SV40 components was tested by inoculation of rabbits and guinea pigs, using the untreated stock, and stocks heated in water and in MgCl2. When the virus stock was heated at 50°C for 30 minutes in water, the adenovirus component lost all of its infectivity and a large part of its antigenicity, while the SV40 component retained both infectivity and antigenicity. When the virus stock was heated in M MgCl2 at 50°C for 30 minutes, the adenovirus component again lost all of its infectivity but it retained all of its antigenicity. However the SV40 component was reduced both in infectivity and in antigenicity.