The positive and negative chemical ionization mass spectra of a series of maytansinoids have been recorded. Unlike electron ionization, in the chemical ionization spectra the high mass ions that are indicative of structure are abundant. Collision activated decomposition produced daughter ions that could be used to confirm proposed structures of characteristic positive and negative ions. Daughter spectra were useful to differentiate structurally similar isomers that had nearly identical mass spectra. Mslms scans were also used to identify three new maytansinoids that were present as minor components in some of the reference standards. Using parent scans, maytansinoids were detected at low levels in simple extracts ofdctinosynnemapreriosum fermentation beers with no sample clean-up.