Restriction endonuclease HindIII was purified from Haemophilus influenzae Rd. Two active fractions, P1 and P2, were obtained in phosphocellulose chromatography. HindIII could be purified completely from the first fraction, P1, by subsequent DEAE-cellulose chromatography. The second fraction, P2, showed HindIII activity higher than that of P1, though it was still contaminated with some minor proteins. The HindIII in P2 fraction showed differences in stability, binding to substrate DNA, electrophoretic mobility, etc., from the HindIII in P1 fraction. It is likely that there are two forms of HindIII in the bacterial cell. The endonuclease HindIII in P2 fraction was finally purified by DNA-cellulose chromatography, though considerable loss of enzymatic activity resulted. Upon infection of the cells with phage T4, the P2 fraction in phosphocellulose chromatography almost disappeared. The presence of two forms of HindIII may be related to bacterial defense against viral infection.