The study was designed to determine the antigenotoxic potential of chlorophyllin (Chl), against the frequency of sister-chromatid exchanges (SCE) produced by benzo[a]pyrene (B[a]P) in vivo. We used the mouse bone marrow test system to measure the effect of a single injection of the compounds: 40 mg/kg of B[a]P, and 1 h later, 1.0, 2.0, 4.0 and 8.0 mg/kg of Chl. As controls we included both chemicals using the dosages mentioned above as well as mineral oil (0.25 mg/kg). The results indicated the following: (1) Chl per se was not genotoxic, showing SCE values in the range of the control level; (2) B[a]P increased the rate of SCEs three times in relation to the basal level; (3) the SCE level produced with B[a]P was diminished by all 4 doses of Chl, but better results were obtained with 2-4 mg/kg, a range which induced Inhibition Indices of 80.9% and 77.5% respectively; (4) the Average Generation Time Index was not modified by the compounds used in the experiment; and (5) the Mitotic Index also showed no significant modification induced by the chemicals, with respect to the control value.