A new method for classifying Mendelian and non-Mendelian antibiotic-resistant mutants in Chlamydomonas reinhardi is proposed. This classification takes into account the cellular site of expression of the resistant phenotype, the location of genetic information for resistance in the cell, and the pattern of inheritance of the different mutants. The classification is consistent with the idea that C. reinhardi contains three protein-synthesizing systems, two of which are sensitive to antibacterial antibiotics and located in the chloroplast and mitochondria, respectively. The information for resistance can be coded either in the chloroplast, the mitochondria, or the nucleus. A new experimental approach to the problem of cellular localization of mutations to antibiotic resistance is prescribed and used for the classification of known resistant mutations.