1. Penicillin in the range of concentration from 250 U/ml. to approximately 2650 U/ml. inhibits the rate of cell division of the fertilized sea urchin egg from 0 to 100 per cent.
2. Penicillin in the same range of concentrations has no effect on the oxygen consumption of the unfertilized or the fertilized eggs.
3. Penicillin is bound by some component of the sea urchin egg in amounts sufficiently large to lower the initial concentration, this binding apparently not being related to the inhibitory action.