Exposure of several spontaneously transformed human lymphoid cell lines to attenuated and virulent types 1 and 2 poliovirus strains led to an initial sharp increase in virus titer followed by a persistent infection. Virus was present as late as 80 passages after infection. The morphology, cell count and viability of virus- and mock-infected cells were similar. Approximately 1 % of the cells scored as infectious centers, and the cultures could be cured by specific antiserum. Virus produced during persistent infections initiated with a virus that readily forms plaques at 40°, Mahoney virus, showed an increase in temperature sensitivity and a decrease in plaque size