Resistance Competition Assay (RCA) of bacteria from the infected legs of mice sampled 3–4.5 hours after treatment of CAB1 infections (mean ± 1 std error). Bacterial resistance was specific to the treatment, phage or streptomycin. Immediate treatment was delivered at the same time as the bacterial inoculation; delayed treatment was delivered 8 hours after inoculation. For treatment with streptomycin and φLH-phage, resistant bacteria have significantly higher RCA values with immediate treatment than with delayed treatment (t-tests, P < 0.002 for φLH-phage; P < 0.005 for streptomycin). With immediate treatment, RCA values of resistant bacteria are significantly higher both for streptomycin and φLH than for φLW (P < 0.005 and P < 0.01, respectively), but the difference between immediate treatment with streptomycin and φLH is not significant (0.05 < P < 0.10). An RCA value of zero indicates that resistant bacteria have no advantage over sensitive bacteria, hence that treatment is presumably ineffective in killing bacteria in the leg; presumed treatment efficacy increases as the RCA increases above zero. Treatment doses were ~108 phage/mouse or 100 μg/gm streptomycin sulfate via an intramuscular injection into a limb. Frequencies of sensitive and resistant bacteria were estimated as in Methods.