Aim: To analyse the growth of Bacillus anthracis during simulations of the UK anthrax vaccine manufacturing process. Methods and Results: Simulated vaccine production runs were performed using the toxigenic, acapsulate Sterne 34F(2) strain of B. anthracis in semidefined medium. After rising during the logarithmic growth phase, the pH of the culture starts to fall at about 18 h from pH 8 center dot 7 to reach < 7 center dot 6 at 26 h, coincident with consumption of glucose and optimal production of protective antigen ( PA; 7 center dot 89 g ml) 1, SD 1 center dot 0) and lethal factor ( LF; 1 center dot 85 g ml) 1, SD 0 center dot 29). No increased breakdown of toxin antigens was seen over the 26 - 32 h period. When glucose was exhausted, amino acids ( principally serine) were utilized as an alternative carbon source. Sporulation was not observed during the 32 h. Conclusions: PA and LF, the principal constituents in the UK anthrax vaccine, undergo little degradation during vaccine fermentation. The vaccine manufacturing process is robust and reproducible. Significance and Impact of the Study: This is the first detailed analysis of the manufacturing process used for the UK acellular anthrax vaccine; insight gained into the process will support continued and safe vaccine manufacture.