Investigations were done to determine whether the replication kinetics of human immunodeficiency virus (HIV)-1 were altered when the virus acquired a set or subsets of five mutations (A62V, V75I, F77L, F116Y, and Q151M) in the pol gene conferring resistance to multiple dideoxynucleosides. In the absence of drugs, the replication rate of all infectious clones generated was comparable to that of wild type HIV-1. However, in the presence of zidovudine or didanosine, the comparative order for replication was HIV-1(62/75/77/116/151) > HIV-1(77/116/151) > HIV-1(75/77/116/151) approximately HIV-1(151), whereas that for drug resistance was HIV-1(75/77/116/151) > HIV-1(62/75/77/116/151) > or = HIV-1(77/116/151) > HIV-1(151). The virologic features of these infectious mutants suggest that HIV-1 develops drug resistance through one or more mutations, which, however, sacrifice replicative capability; then it finally acquires optimal replication competence by additional mutations when the multi-dideoxynucleoside-resistant mutant emerges.