The structure and stereochemistry of neplanocin A was determined on the basis of its spectral and chemical evidences as [1R-(la,2a,313)]-3-(6-amino-9H-purin-9-yl)-5-(hydroxymethyl)-4-cyclopentene-l,2-diol.For the final proof of the structure, X-ray crystallographic analysis of neplanocin A was carried out.In a previous papery, the producing organism, isolation and characterization of neplanocin A, a new antitumor antibiotic from the culture filtrate of Ampullariella regularis A11079, were described.This antibiotic exhibits significant antitumor activity against L1210 leukemia in mice and represents a new member of the nucleoside family from natural sources. This paper deals with the structural elucidation of neplanocin A.Structural Elucidation of Neplanocin A Neplanocin A (1), C11H13N5O3, displayed UV maxima at 262 (log a 4.20, water, pH 7), 262 (log a 4.19, pH 11), and 260 nm (log a 4.18, pH 2). The UV spectra, therefore, resembled those of adenine derivatives, especially adenosine. The IR spectrum of 11) also similar to that of adenosine, showed absorptions at 1650, 1600 and 1570 cm-1 (vC=C, vC=N), typical of a purine moiety and at 3100~3400 cm-1 (vOH.NH)• The only difference between the spectrum of 1 and that of adenosine was the presence of the additional absorption of 1640 cm-1 (vC=C) in 1. In the mass spectrum (El, 70 eV) of 1, the ion peak at m /z 136 supported the presence of an adenine moiety. This ion occurred by the cleavage of the bond between the base and the sugar moiety accompanied with the transfer of two hydrogens to the base2).The 1H-NMR spectrum of compound 11) revealed two very sharp singlets at 6 8.16 and 8.09 due to the aromatic protons at C(2) and C(8) of the adenine moiety and a broad peak (2H) at 5 7.21 due to an amino group. The remaining protons of the spectrum consisted of a series of peaks from 6 4.1 to 5.8 (9H). The addition of a small amount of acetic acid-d4 to 1 in dimethylsulfoxide-d6 (Fig. 1) (1 H) at 6 4.34 in 1 were assumed to be due to a hydroxymethyl proton and two hydroxymethine protons since the splitting of the respective protons changed by the addition of acetic acid-d4. To assign each of the signals, decoupling experiments were carried out as summarized in Table 1. In addition, the spec-