Monochloramine preparedin situby first adding chlorine to a suspension of microorganisms, followed by subsequent addition of ammonia, inactivated the MS2 coliphage more rapidly than did exposure of phage to 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 of MS2 to free chlorine before the addition of ammonia. The average 99% CT value of MS2 when exposed to free chlorine was 1.3 and 1.1 at 5 and 15 °C, respectively. The average 99% CT values of MS2 briefly exposed to the combined action of free chlorine followed by the addition of ammonia to form monochloraminein situwere 19.3 and 1.5 at 5 and 15 °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-h contact time. Inactivation of MS2 by monochloramine was more rapid at 15 than at 5 °C and when the chlorine to nitrogen weight ratio was 5:1 compared with 3:1. Monochloramine was a more efficient inactivating agent for the coliformsEscherichia coliandKlebsiella pneumoniaethan it was for the MS2 coliphage.Key words: chlorine, monochloramine, virus, bacteria, disinfection.