The effects of media composition on growth parameters, total protein production (including delta-endotoxin) and its relation to the biological properties of Bacillus thuringiensis var. israelensis was investigated. The replacement of glucose by glycerol as the carbon source yielded higher concentrations of delta-endotoxin. This increase in toxicity was associated with increased amounts of the 130 kDa polypeptide fraction. The biocide activity was not related to haemolytic activity, specific growth rate nor spore count.