Actinophage active against both neutrophilic and acidophilic streptomycetes were isolated from a variety of soils by a specific enrichment method. No phage were detected in soils with a pH below 6·0, despite the presence of acidophilic streptomycetes in these soils. However, some phage isolated from neutral soils were able to lyse acidophilic streptomycetes at pH 5·5. Selected actinophage were shown to be stable between pH 5·5 and 9·0 in sterile soils and broth, but were rapidly inactivated at lower or higher pH values. Survival of these phage was good in neutral soils, but negligible in acidic soils. This suggested that free phage were unable to remain infective in acidic soils, thus at least partially accounting for the failure to isolate them. Acidity was shown to have variable effects on several stages of phage replication including adsorption, penetration and the length of the latent period. The latter effect was directly related to the metabolic activity of the host.

Over 60% of streptomycete phage, isolated from soil, were adsorbed by unsubstituted or sodium-treated kaolin, when suspended in nutrient broth, but less phage were adsorbed by sodium and calcium montmorillonite or by calcium- and aluminium-treated kaolin. The montmorillonites did, however, partially protect one of the phage from damage during experimental manipulations. Adsorption to kaolin was not influenced by pH and most of the adsorbed phage retained their infectivity. Attempts to obtain free infectious phage by desorption were unsuccessful. The pH stabilities of adsorbed and free phage differed: 50% inactivation of phage adsorbed to kaolin occurred at pH 6·1 whereas for free phage 50% inactivation occurred at pH 4·9. This suggested that ΔpH (pHb–pHs) was about 1·2 units, i.e. similar to previous estimates based on the activity of enzymes adsorbed to kaolin.