After treatment of the wild-type strain 975 h+ with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) mutants able to grow on YEP-medium containing 15 mM CdCl2 were isolated. Mutant R17 showed a specific resistance to Cd, but no increased tolerance to Pb(NO3)2, CuSO4 or ZnCl2. No higher content of Cd-binding phytochelatins could be detected, but the mutant accumulated lower levels of Cd than the corresponding wild-type. No change, however, was observed in the accumulation of Cu. Under the influence of Cd, the respiratory activity of mutant R 17 is reduced less than in the wild-type. The mechanism of resistance could be an increased secretion or a decreased uptake of Cd.