Simian virus 40 gained notoriety in the 1960s because it was found to be a contaminant of polio and adenovirus vaccines that had been administered to millions of healthy individuals worldwide. The public health implications of this revelation provided the initial impetus for an in-depth study of SV40 biology. Later work showed that SV40 DNA sequences as well as infectious virus are in fact found in human tumors and may have contributed to oncog- esis. It also turned out that SV40 uses mostly cellular machinery to carry out many steps in viral infection, which makes it a powerful probe for examining many fundamental questions in eukaryotic molecular biology. SV40 Pro- cols consolidates a number of well-tested step-by-step techniques in one v- ume; experts with hands-on experience in particular methods give detailed accounts of their optimized experimental protocols, so that the beginner, as well as more experienced researchers, may readily overcome problems of ambiguity often present in the literature. As with other DNA tumor viruses, the response of cultured cells to SV40 infection depends upon the species being infected. Monkey cells s- port virus production, which leads to their death, whereas rodent cells p- duce only the early proteins and acquire a transformed phenotype. Thus, SV40 Protocols is organized in two sections. The first relates to assays of the lytic cycle of the virus, and the second deals with transformation.