This is the second of two papers describing the biology of the naturally occurring RNA tumor viruses (oncoviruses). It will appear in two parts. In the first paper [Cancer Investigation 1(1):67–83 (1983)] the general properties of this class of viruses and the biology of the retroviruses of the“lower”vertebrates was discussed. In this paper the oncoviruses of the“higher”animals are described. Part one deals with cat retroviruses.