An in vivo assay was previously developed for detecting forward mutations in human immunodeficiency virus type 1 (HIV-1) in a single cycle of replication. This system uses the lacZα peptide gene as a reporter for mutations, and allows for the rates and types of mutations that occur to be determined. The forward mutation rate for HIV-1 in HeLa cells was found to be 3 x 10-5 mutations per target base pair per cycle. To test whether the mutation rate was influenced by cell type, the mutation rate of HIV-1 in CEM- A cells, a T lymphoid cell line, was determined. The mutation rate of HIV-1 reverse transcription in CEM-A cells was found to be 4 x 10-5 mutations per target base pair per cycle. The number and types of mutations observed were similar to that in HeLa cells. Specifically, base substitution mutations predominated, and G-to-A transition mutations were the most common base substitution. G-to-A hypermutants were also characterized. The difference in HIV-1 mutation rate between HeLa and CEM-A cells was not significant, indicating that the accuracy of HIV-1 reverse transcription is comparable in both the HeLa and CEM-A cell lines.