Alanosine (2-amino-3-[hydroxynitrosoamino] propionic acid) an extracellular antitumor product of <i>Streptomyces </i>inhibits the <i>in vitro </i>morphogenesis of mouse molar tooth germs. Organ cultures prepared from 15- and 16-day-old C5 7BL/6J embryos were maintained on agar-solidified BME, supplemented with glutamine, gentamicin and fetal calf serum, in an incubation atmosphere of 5% CO<sub>2 </sub>and 95% air. At a concentration of 16 <i>µ</i>g/ml, alanosine severly inhibited growth of the explants and development of the tooth germs. Tooth germ inhibition was expressed as abnormal, or a lack of, folding of the dentino-enamel junction, disorientation of preameloblasts, negligible numbers of tooth germ cells in mitosis and failure of 2nd molar tooth germs to develop. These explants, after 4 days in culture, contrasted with larger explants, produced during the same culture period on control medium, exhibiting normally developing 1st and 2nd molar tooth germs with large numbers of cells in mitosis. The effects of the drug on the tooth germs were partially reversed by allowing the explants to recover on control media. Alanosine inhibits the conversion of inosine monophosphate to adenosine monophosphate, at the first step of a two-step conversion, in the <i>de novo </i>pathway of adenine nucleotide biosynthesis. The cytological and morphogenetic inhibition of tooth germ morphogenesis by alanosine can be partially prevented by the simultaneous application of adenine. These observations suggest that: (1) tooth germ development depends on adenine nucleotide synthesis, and (2) salvage as well as <i>de novo </i>adenine nucleotide biosynthetic enzymes function in normal tooth development.