With the diminishing supply of the human fetal lung WI-38 cell strain, a replacement for viral isolation is needed. Two candidates are the human fetal lung strains MRC-5 and IMR-90. A comparison of WI-38, MRC-5, and IMR-90 was performed to evaluate efficiency and speed of viral isolation, clarity of cytophatic effect, and ease of growing the cells. The inocula were clinical specimens rather than tissue culture-adapted isolates. Frozen samples of 46 specimens that had previously yielded an isolate on WI-38 were thawed and inoculated onto WI-38, MRC-5, and IMR-90 cells. In addition, 95 freshly taken clinical specimens uf undetermined infectivity were inoculated onto the cell strains. Viral recovery rates were similar on all three strains, as were the appearance and speed of onset of the cytophatic effect. MRC-5 and WI-38 cells remained healthy until generation 36, whereas IMR-90 cells went into crisis by generation 20. The longer life span of the MRC-5 cells makes them more suitable than IMR-90 cells to replace the WI-38 strain for routine use in viral diagnosis.