An antibacterial factor, aplysianin E, was purified from the eggs of a sea hare, Aplysia kurodai. Purified aplysianin E was a glycoprotein of 250 kilo daltons consisting of 3 subunits, and showed both antibacterial and antineoplastic activities. The two activities were lost in parallel on heating and at low and high pH. This factor was half-maximally active for gram-positive and -negative bacteria at 0.12-3.3 micrograms/ml and its action was not bactericidal but bacteriostatic. Aplysianin E did not induce morphological elongation of bacteria or their release of adenosine triphosphate (ATP), but it completely inhibited the syntheses of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) by E. coli within 10 min. These results suggest that aplysianin E, found in an invertebrate, the sea hare, is a new antibacterial protein and that it exerts its action by inhibiting nucleic acid synthesis, as a DNA-inhibiting chemotherapeutic drug does.