1. The efficiency of plating of phage 13 on the semi-resistant strain Ps. LIII-3bi decreases as the initial bacterial population on the plate is diminished. Addition of Ps. LII extract to the phage before plating with low bacterial concentrations increases the plating efficiency (exhibition effect).

2. An extract of a strain Ps. LIII-lb which is fully sensitive to phage 13 also causes exhibition of this phage on Ps. LIII-3bi.

3. It is suggested that dissolved phage 13 receptors present in extracts of Ps. LII and Ps. LIII-lb have an affinity not only for the phage but also for suitable bacteria and thus cause exhibition. Inhibition (e.g. of phage 14 by the Ps. LII extract) is due to phage receptors having no tendency to adsorb to bacteria.

4. The phage-exhibiting and inhibiting properties of a freeze-dried extract of Ps. LII active when dissolved in 0.85 % NaCl are lost if the solid is dissolved in water and the solution so obtained further diluted with 0.85 % NaCl. Activity is recovered slowly if NaCl is added to the aqueous solution, instantly if the solution is freeze-dried and the residue dissolved in 0.85 % NaCl.

5. The inhibitory properties of the Ps. LII extract for phage 14 are more sensitive to heat and acidity or alkalinity than the exhibitory properties for phage 13.

6. The absence of cross-precipitin reactions between Ps. LII and Ps. LUI-lb suggests that the phage 13 receptors of the two organisms are non-antigenic or different. I am grateful to Prof. M. van den Ende and my colleagues in this Unit for helpful discussions.