Repeated intraven. injections of tyrosinase modified the properties of rabbit serum. Serum prepared from active tyrosinase inhibited a strongly active tyrosinase preparation, but reactivated a weak or even feebly active tyrosinase. Serum from very feeble or practically inactive tyrosinase had no effect on active tyrosinase, but reactivated a feeble or practically inactive tyrosinase. The explanation may be that 2 antibodies are generated, specific, respectively, to the enzyme and to a presumed antagonistic factor in the enzyme preparations. Injection of the inhibitory constituent of an enzyme preparation might perhaps be a general method for activation of an enzyme, by means of the antibodies produced.