In its integrated form. SV-40 DNA offers an opportunity to observe the behavior of what is in effect a viral genome within a cellular genome, with transcriptional and translational products that can be clearly distinguished from those of the host cell. Exposure of SV40-transformed human fibroblasts to a 60 Hz continuous-wave sinusoidal electromagnetic (EM) field resulted in increased levels of virally derived mRNA and protein of large T-antigen. These findings provide evidence that a foreign DNA integrated into cells can be affected by EM fields under conditions known to cause increased transcripts from endogenous cellular genes.