TABLE 2.

Recovery of viruses added to tap water as determined by the most probable number approach (one 50-ml preparation, five 10-ml preparations, and five 1-ml preparations) when the method described here was used^a

Sample	No. of PFU addedb	Most probable no. (100 ml)	% Recoveryc
1	40	91	227
2	22	17	77
3	30	53	177
4	34	91	268
5	32	53	166
6	28	53	189
7	10	4	40
8	26	53	204
Mean	27.7	51.8	168.5
SD	9.0	30.6	77.0

^aThe pH values of virus suspensions in water were decreased to 3.5. 

^bThe number of viruses added per 100 ml was determined by performing a plaque assay with confluent cell monolayers. 

^cPercentage of viruses recovered after filtration compared to the number of viruses determined by the plaque assay method performed with confluent cell monolayers.