Electron microscopic examination of hepatoma tissue culture (HTC) cells revealed low numbers of intracisternal type A particles (IAP) and type C viruses. Exposure of HTC cells to either 10(-4) or 10(-5) M5-bromo-2'-deoxyuridine (BUdR) caused a more than 50-fold increase in the number of IAP observed with the electron microscope. The number of IAP increased after only 2 days of growth in 10(-5) M BUdR, whereas 3 days of growth in 10(-4) m BUdR were necessary to observe an increase. A 2-day pulse of 10(-4) M BUdR was also sufficient to cause the increase in type A particles, provided the cells were continued in culture for another 2 days. Unlike many other cell lines, HTC cells treated with BUdR did not show an increase in type C viruses. This conclusion was based on the observations that BUdR treatment caused no detectable increase in extracellular particulate viral reverse transcriptase or in viral RNA complementary to a DNA probe made to a rat endogenous type C virus (RaLV).