The effect of aphidicolin, a specific inhibitor of DNA polymerases alpha and delta, on recovery and repair of X-irradiated human fibroblasts was investigated. Aphidicolin concentrations from 0.5 to 5 micrograms/ml reduce DNA synthesis in exponentially growing cells after short incubation periods (1 h), longer incubation times (24 h) enhance this effect. Colony forming ability of unirradiated fibroblasts, treated in confluent growth state with aphidicolin, was not reduced. After incubation with aphidicolin contact-inhibited fibroblasts show reduced PLD recovery. For the short preincubation time (1 microgram/ml) the effect depends on aphidicolin concentration, for the long preincubation period the effect is independent of concentration. DNA double-strand break induction and rejoining was investigated after combined action of aphidicolin and X-rays using the neutral filter elution technique. The yield of initial DNA double-strand breaks was not affected by aphidicolin. After a 90-min recovery time, however, DNA double-strand break rejoining was markedly reduced. This effect was dependent on concentration for the short incubation time (1 h) but not for the long preincubation (24 h).