Mass spectrometry is being employed as a structural and analytical aid in investigations of the antibiotics dermostatin, candimycin, berninamycin, scopafungin, hamycin, antiamoebin, aureofungin, and aminocyclitol antibiotics, as well as of the Ormosia and Erythrina alkaloids, soybean phytoalexin, and complex mixtures of compounds from marine organisms. Mass spectrometry, using stable isotopes, is also beeng applied to studies of the biosyntheses of the antibiotics neomycins B and C, chloramphenicol, nybomycin, the streptovaricins, and geldanamycin, as well as to biosyntheses of the para-sympathomimetic fungal metabolite slaframine. Mass spectral fragmentations of aromatic compounds are being investigated using compounds specifically labeled with stable isotopes.