Structure elucidation of natural products including the absolute configuration is a complex task that involves different analytical methods like mass spectrometry, NMR spectroscopy, and chemical derivation, which are usually performed after the isolation of the compound of interest. Here, a combination of stable isotope labeling of Photorhabdus and Xenorhabdus strains and their transaminase mutants followed by detailed MS analysis enabled the structure elucidation of novel cyclopeptides named GameXPeptides including their absolute configuration in crude extracts without their actual isolation. Game over—structure solved: A combination of labeling experiments with mass spectrometry results in the reliable determination of the sum formula, the nature of the building blocks, and (for peptide natural products) also the determination of the absolute configuration as exemplified for the novel natural products GameXPeptide A–D (see scheme).