The determination of growth rates from detection times measured in an automatic micro-well plate photometer is described. With triplicate cultures of each of four different inoculum concentrations, the growth rate of Salmonella typhimuriumwas determined with a coefficient of variation of 2%. The minimum inhibitory concentration (MIC) of diacetyl for Salmonella typhimurium at 30°C, pH 5.0-6.0, was 1.4 mmol l - 1 when the initial inoculum was 10 4 cfu ml - 1 , but was lower for a smaller inoculum. Sublethal diacetyl concentrations were found to reduce the specific growth rate of Salmonella typhimurium. pH values over the range pH 5.0-6.5 had little effect on the antimicrobial activity of diacetyl.