1. The action of phenol on Bact. coli has been studied quantitatively in detail under such conditions that unfavourable circumstances other than the presence of the germicide were as far as possible eliminated. Briefly, this involved the addition of phenol solution directly to a large volume of culture developed under rigidly standardized conditions.
2. When the mortality exceeded 95% the variation between the numbers of colonies on replicate plates, when determining the numbers of survivors, was often excessive.
3. Under the conditions used the death-rate was not constant. First, there was a phase (1) of slow but increasing death-rate which merged gradually into a phase (2) of approximately constant rate which was also the maximum for a given phenol concentration. There are indications that the death-rate may decline towards the extreme end of the disinfection process.
4. The virtual extinction time (the time taken for the survivors to fall to 1 per ml. as determined by extrapolation of the log survivors-time curve) increased as the phenol concentration decreased, as also did the duration of phase 1. The latter occupied about 60% of the virtual extinction time when the concentration was 3·48 g. phenol per 1., but with rising concentration its duration decreased at a greater rate than the virtual extinction time so that at about 7 g. per 1. phase 1 was very short though still detectable. The standard errors of the virtual extinction times varied between ± 1·5% and ± 5·43%.
5. Using the probit method it has been shown that at low concentrations of phenol the distribution of resistance of Bact. coli is approximately normal when the resistance is measured in terms of the survival times.
6. The distribution of resistance was found to be normal at all concentrations of phenol used, when that resistance was measured in units of the logarithms of the survival times, with the exception that at the lower concentrations there was a sudden change in the slope of the probit-log survival-time line at about 20% mortality. This change became less marked with rising concentration and was probably absent at 8·00 g. phenol per 1.
7. After conversion into the dosage-mortality form, the data reveal that about 70% of the cells show an approximately normal distribution of resistance when the latter is expressed in terms of the dosage survived for a fixed time, provided that the exposure time is less than 50 min.
8. The distribution tends to be normal for longer exposure times when the resistance is measured in terms of the logarithms of the dosages survived. On this basis a change of slope of the probit-log dosage line is observed as in the case when resistance is measured in terms of the logarithms of survival times.