RNA-RNA recombination is thought to occur preferentially at certain selected sites and in only a few RNA viruses; the mechanism for these restrictions is unknown. In this paper we report the development of a recombination assay for coronavirus, using polymerase chain reaction, in the absence of selection pressure. Our results showed that within a 1-kb region of the peplomer gene, RNA recombination occurred at almost every potential crossover site. Thus, coronavirus RNA recombination appears to be more random than previously realized. However, after serial passages of the recombinant viruses in tissue culture, the recombination sites among the progeny viruses became clustered in the region which contains the previously reported "hot spot" for coronavirus recombination. These results suggest that RNA recombination is common and random in nature, but only certain recombinants can be selected. Thus, the presence of recombinational "hot spots" for coronavirus or other RNA viruses most likely resulted from selection of certain recombinant viruses and not restriction on the occurrence of RNA recombination. The failure to detect recombinants in other RNA viruses may therefore be due to unfavorable properties of recombinant viruses. This approach can be used to detect recombinants in these viruses.