The isolation of a novel tetracyclic taxane, taxinine M [I], from Taxus brevifolia bark and its structure elucidation by spectroscopic methods are reported. The large-scale isolation of taxol (1-3) from Taxus brevifolia Nutt. (Taxaceae) bark has provided an opportunity to examine the many related taxanes present in abundance in this plant. We report here the structure elucidation of a novel taxane which elutes near taxol in the large-scale isolation process. The fab mass spectrum of the title compound showed a sodium adduct ion at m/z 709, which on high resolution measurement corresponded to a formula of C35H42O14Na. A second ion corresponding to loss of H2O from the protonated molecular ion was measured as C35H41O13. Thus the mol wt of the compound was determined as 686, and the molecular formula as C35H42O14. The unsaturation number of 15 could be accounted for by a tetracyclic structure with one isolated double bond, a single unsubstituted benzene, and six carbonyls. The 13C-nmr spectrum showed one of these carbonyls to be a ketone and the rest to be esters. Close analysis led to the conclusion that there were four acetates present and one benzoate. A puzzling point in the 1H-nmr spectrum was the paucity of methyl singlets compared to other known taxanes. From the COSY spectrum it was possible to discern structure fragments which accounted for several parts of the molecule. An inverse detection one-bond heteronuclear correlation experiment (HMQC) (4) connected most of these protons to specific proton-bearing carbons. The single benzoate accounted for all but two of the carbon lines in the range 110 to 150 ppm. Four singlets (91, 80, 49.7, 49.6 ppm) were also unaccounted for in the sp3 hybridization range. The long-range heteronuclear experiment HMBC (4) was used to locate the positions and connectivity of these singlets, as well as the points of ester attachment. The most unusual features of the structure are the Me-19 benzoate and the cyclized Me-16 unit. Detailed results of the correlation experiments are presented in Tables 1 and 2 and Figure 1.