SV40 is capable of infecting humans, although its association with human diseases remains controversial. Recently, a subgenomic SV40 DNA sequence was detected by polymerase chain reaction (PCR) in certain types of human tumour tissue as well as in normal pituitary tissue. However, due to the limited DNA sequence information that was obtained in those experiments, SV40 could not be authenticated, and it was uncertain whether a related or hybrid virus (or endogenous DNA) accounted for the PCR-amplified DNA. To gain more insight into these observations, we are experimenting with PCR primers directed at various sites of the SV40 genome, as well as with various parameters of the PCR assay. In this communication, we describe methodology we currently use in our studies and discuss problems associated with the PCR detection of SV40 in various types of samples.