Localization of sulfomucopolysaccharides in developing teeth of Swiss albino mice was detected by S35 autoradiography and histochemistry. A positive correlation was found to exist between autoradiographic and histochemical data with regard to the localization of sulfomucopolysaccharides. Autoradiography, however, revealed some sites of localization which were not detectable by histochemistry, namely, the odontoblasts and stratum intermedium. Fetuses which received the isotope via maternal injection at the cap stage of tooth development and were sacrificed after 2 hours of isotope action displayed rapid incorporation of the isotope in the components of the dental papilla. In the enamel organ, however, only moderate activity was recorded. When the time interval between injection and sacrifice of the experimental animals was increased to 20 hours, intense activity was observed in the enamel organ. With progressively longer intervals between injection and sacrifice, S35 was demonstrable first in odontoblasts and later in the predentin. This occurred as a band or active zone which migrated toward the dentino-enamel junction. With the increasing intervals between injection and sacrifice, first the odontoblasts were active, then predentin was active while the odontoblasts became reduced in activity, after which the dentin matrix gained activity while the predentin decreased somewhat in activity. This pattern is consistent with appositional growth. A linear band of activity was not observed in the enamel matrix; rather, the activity was present as a diffuse stippling over a relatively large area of the matrix. The sulfomucopolysaccharide which existed in dentin matrix was postulated to have originated from the cells of both the odontoblastic layer and the dental papilla.