Abstract
The SV40 DNA that was generated by undiluted passaging of the virus was analysed by agarose gel electrophoresis. Nine bands of virus DNA were distinguished and each band contained a specific size class of DNA, all shorter than the complete SV40 genome as was determined by electron microscopy measurements. A difference of 2% in length, about 100 base pairs, resulted in a clear band splitting. Two sets of undiluted passaging were established and the defective DNA in the two sets had both different and similar size classes varying in length from 96% to 73% of the unit length SV40 DNA.