Human lymphocytes were exposed in G1 to mitomycin C (2.5 microM for 2 h) and harvested at 3-h intervals from 48 to 84 h after stimulation. All cultures were also post-treated in G2 with caffeine (2 mM). Different types of chromosomal aberrations were scored in the first division metaphases. Caffeine increases all chromosome aberration types by promoting a premature mitosis of damaged cells. However, when the frequency of damaged cells is not affected by the caffeine post-treatment, a reduction of the frequency of the exchange-type aberrations was shown. The possibility that caffeine interferes with some mechanism of G2 repair is discussed.