The invention relates to a mass-spectrometric method to determine microbial resistances to antibiotics, in which the microbes are cultured in a medium comprising an antibiotic, and a mass spectrum of the microbes is acquired after they have been cultured. The method is characterized by the fact that any microbial growth taking place during the culture is mass-spectrometrically determined with the aid of a reference substance, which is added in a dosed amount and is co-measured in the mass spectrum, wherein a growth in microbes indicates the resistance to the antibiotic.