Artificial carious lesions were produced by a gelatin/lactic acid system in deciduous bovine, permanent bovine, deciduous human and permanent human enamel over periods up to 8 days, and the progress measured by dye imbibition studies. Artificial carious lesions were produced in bovine, ovine and human permanent enamel using a diphosphonate/lactic acid system over periods up to 30 days, progress was assessed by depth measurement, and overall diffusion coefficients were calculated. The rate of lesion progress was linear with the square root of time for both systems of assessment. Lesion progress as measured by dye imbibition was in the ratio 3.7: 2.9: 1.5:1.0 for bovine deciduous: bovine permanent: human deciduous: human permanent, respectively. Bovine permanent, ovine permanent and abraded bovine permanent lesions progressed at equal rates and in the ratio 3.0: 1.0 to human permanent enamel, as measured by overall diffusion coefficients. The results provide an approximate basis for the use of bovine, ovine and human artificial carious lesion data in the quantitative discussion of the caries process.