Monochloramine prepared in situ by first adding chlorine to a suspension of microorganisms, followed by subsequent addition of ammonia inactivated the MS2 coliphage more rapidly than did monochloramine prepared either by adding chlorine to ammonia or by adding chlorine and ammonia simultaneously. The rapid viral inactivation was apparently due to the exposure MS2 to free chlorine before the addition of ammonia. The average 99% CT value of MS2 to free chlorine was 1.3 and 1.1 at 5 deg C and 15 deg C respectively. The average 99% CT values of MS2 to the combined action of brief chlorine exposure followed by the addition of ammonia to form monochloramine were 19.3 and 1.5 at 5 deg C and 15 deg C respectively. No 99% CT values were calculated for the inactivation of MS2 with preformed monochloramine because less than 1 log (90%) of inactivation occurred during a 4 hour contact time. Inactivation of MS2 by monochloramine was more rapid at 15 deg C than at 5 deg C and when the chlorine to nitrogen weight ratio was 5:1 compared to 3:1. Monochloramine was a more efficient inactivating agent for the coliforms, Escherichia coli and Klebsiella pneumoniae than it was for the MS2 coliphage.