A filtration method has been developed which can be used to detect and enumerate phage in low concentrations directly from solution without the need for prior concentration. In this method, a known volume of the phage solution is mixed with a suitable host solution. Samples are filtered through membrane filters; the filter is removed and incubated, and after 24 hr the resultant plaques are counted and the titer is calculated. Escherichia coli B and the coliphage T2 were used in these studies. Host cultures less than 12 hr old produced the best results. Approximately 1010 host organisms must be present in the sample taken for filtration. To avoid phage reproduction, all steps prior to filtration must be done in less than 45 min. The method was compared with the soft-agar technique and was shown to be less precise but able to measure phage in lower concentrations.