Cultures of human immunodeficiency virus type 1 (HIV-1) provided a model for the study of mutations in the absence of host antibodies. Replicate cultures of biological and molecular clones of HIV-1 were passaged weekly for 30 or 34 weeks. Eight regions of HIV-1 genomic RNA were analyzed by means of single-strand conformation polymorphism analysis and nucleotide sequencing. Six mutations were detected in the biological clones. Two were G-->A substitutions. The frequency of mutations was higher in V1 compared to that in other regions (P = 0.01). Three mutations involved loss of potential glycosylation sites in V1. These results show that mutations in the viral genome may result from selection by factors other than host immune pressures.