Haemocin, a bacteriocin produced by Haemophilus influenzae b, caused pronounced cell elongation of the sensitive strains H. influenzae Rd and Escherichia coli CR34 thyA. In the sensitive strains, haemocin markedly inhibited the synthesis of deoxyribonucleic acid (DNA) but did not affect either ribonucleic acid or protein synthesis. The observed inhibition in synthesis of DNA was not due to degradation of preexisting DNA. The data suggest that haemocin may specifically inhibit DNA synthesis.