In order to study the repair capacity of fertilized hamster eggs for the lesions present or induced in human sperm, we have examined the potentiating effect of caffeine, a DNA repair inhibitor, on the frequency and types of sperm chromosome aberrations. Sperm samples were donated by an individual treated with chemotherapy for a testicular cancer 3 years previously. Exposure of spermatozoa and inseminated oocytes to caffeine led to an increase of sperm chromosome aberrations, indicating that the damage to human sperm can be repaired in untreated hamster egg cytoplasm. The potentiating effect of caffeine was mainly reflected in an increase of unrejoined aberrations, indicating that the formation of chromosomal rearrangements is also inhibited. Since both chromatid-type and chromosome-type aberrations increase after treatment with caffeine, damage to human sperm can probably be repaired inside the hamster egg cytoplasm by pre- and post-replication repair mechanisms.