Method for regenerating deactivated microorganisms

In a method for reactivating the methane oxidizability of microbial cells of a methane-utilizing bacteria which have partly or wholly lost their methane oxidizability, the improvement which comprises reactivating said methane-utilizing bacteria in an reactivation solution containing a carbon source selected from the group consisting of not less than 10 n mol/minute mg of cells methane, about 10 to 600 n mol/minute mg of cells methanol and about 10 to 600 n mol/minute mg of cells formaldehyde or a mixture thereof, while supplying thereto a not less than 1 n mol/minute mg of cells nitrogen source, a not less than 0.02 n mol/minute mg of cells sulfur source and oxygen.