Various experiments showed that plastic culture boxes may be sterilized by formaldehyde vapour in an exsiccator. Detectable quanties of formaldehyde were no longer found to be present when the dishes had been stored at room temperature for 24 hours after sterilization. A striking fature in these experiments consisted in the fact that colonies failed to grow on those agar plates, the surfaces of which had been in contact with formaldehyde vapour. The dishes used should be meticulously cleaned as studies showed that complete sterilization was not achieved when bacteria were present in or on agar. When Petri dishes cleansed in alcohol and then sterilized were used in assessing aerobic bacterial counts in minced meat by the mixed plate technique, significantly higher average numbers of colonies were recorded than was the case when commercially available sterilized Petri dishes were used. When the streak plate method was adopted, however, no significant differences were observed. When all the required manoeuvres are performed with meticulous care, the method is a practicable one ensuring that plastic culture boxes may be used again several times.