Slowly growing mutants have hampered the assessment of the mutagenic effect of chemicals. A new, in situ protocol allows for the inclusion of slowly growing mutants in the measurement of the mutagenic response. Inclusion of these slowly growing mutants increases the measurement of the mutation rate by 50-fold over old techniques. These new techniques allow for a more accurate measurement of the molecular lesions induced by chemicals and these lesions are being related to the DNA adduct induced by these chemicals.