The spontaneous hprt mutant clone SP5, derived from V79 Chinese hamster cells, was shown to exhibit a duplication of approximately 2 kb, including exon 2 and its flanking intron sequences, inserted into the intron 1 sequence of the hprt gene. The most striking feature of SP5 is that this clone is quite unstable, demonstrating an extremely high spontaneous reversion frequency. Molecular analysis of 25 independent revertant clones of SP5 indicated that they arose after precise deletion of the duplicated fragment in the hprt gene. Reversion of SP5 could be induced by agents which damage DNA by different mechanisms, but there was no correlation with induction of the forward mutations. Based on these results, we suggest that intrachromosomal recombination must be responsible for the spontaneous reversion of SP5. Genetic recombination in somatic cells has been suggested to be involved in the multistep process of carcinogenesis. Since the ability to induce intrachromosomal recombination in yeast has been shown to be highly correlated with non-mutagenic as well as mutagenic carcinogens, it is of great interest to investigate similar systems in mammalian cells. The SP5 cell line may be unique for such a purpose, since this mutant clone contains an endogenic marker for studying the process of intrachromosomal recombination.