SUMMARY: The time taken for agar plates to reach incubator temperature under different conditions was determined by thermocouple measurements. From 1 to 5 hr. was taken to reach within 1° of the incubator temperature, according to their mass, position, stacking and original temperature. Delay in heating slowed the growth of organisms and produced larger inhibition zones due to diffusing antiseptics. This leads to errors in sensitivity and thus in assay work unless technique is standardized to overcome these differences. Deviations from the calculations resulting from the formula of Cooper & Woodman (1946) can be overcome by such standardization.

An appendix discusses the theory of inhibition zones and compares the formulae proposed by Cooper & Woodman (1946), by Vesterdal (1947) and by Mitchison & Spicer (1949).


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