By means of the electron spin resonance technique it is possible to detect carbonate-related paramagnetic centers in human tooth enamel. Several arguments indicate that the radical formed after X-ray irradiation is a CO2- rather than a CO(3)3- center. There is experimental evidence that the distribution of the orientation of those centers is related to caries-susceptibility, but care has to be taken with regard to the interpretation of the results formulated in the literature. In our own preliminary research, we have studied the ESR-spectra of both bovine and human tooth enamel. As a model for human tooth enamel, the study of bovine enamel was unsatisfactory, because the signals were visibly different from the ones obtained on human tooth enamel. The reproducibility of the ESR-signals in human tooth enamel was carefully studied and found to be excellent. We also investigated the effect of heating these samples, and noted a drastic change in the spectra. Our numerical values for the R-parameter, as introduced by Cevc et al. (1976, 1980), are considerably lower than the ones they published. Clearly, additional information concerning the sample-preparation-technique and the etiological factors which determine the R-value, should be gathered.