The effects of deoxynucleoside triphosphate (dNTP) imbalances on the fidelity of human immunodeficiency virus type 1 (HIV-1) replication were investigated. Using detergent permeabilized virions and biased dNTP concentrations different types of hypermutants were readily produced. However, the mutant spectrum was different from naturally occurring hypermutants demonstrating that the host cell may restrict variation. Using a genetic screen based on the blue/white beta-galactosidase complementation assay, G --> A hypermutants were recovered from HIV-infected thymidine treated U937 cells. Furthermore, hypermutants were recovered from 1 to 2% of resting or activated peripheral blood mononuclear cells indicating that small proportions of primary cells had distorted intracellular [dTTP] and [dCTP]. Such imbalances may underlie a proportion of somatic and germline point mutations and shape to some extent the evolution of mammalian and viral genomes.