1. A modified rapid plaque-counting procedure is described, whereby staphylococcal phage host-cell suspensions in agar are spread over constant areas on glass slides and are incubated in moist chambers.
2. Statistical analysis indicated a precision of ±8.2 for means of sets of eight slides incubated at 37°C. and ±7.6 for means of sets of eight incubated at 28°C.
3. With the staphylococcus system, a temperature of 28°C. yielded significantly higher counts than at 37°C.
4. Counts made after 4 hours may be used as an estimate of the final plaque count bearing in mind the fact that the yield at this time is significantly less than at 24 hours.
5. The data presented compared favorably with the results obtained by the plate method used by Hershey, and suggest that a greater precision may be obtained by this method—at least for the staph. K phage system.