The expression of virulence determinants by Bordetella pertussis was studied during growth in batch and continuous culture. The addition of 2,6-0-dimethyl βcyclodextrin (CD) reduced the lag phase of batch cultures, increased the maximum culture density in shaken cultures and increased production of lymphocytosis promoting factor (LPF) and filamentous haemagglutinin (FHA). CD was not a nutrient but in its presence glutathione was no longer required. Purified FHA was stable when incubated statically but lost activity on shaking. CD had a stabilising effect on FHA incubated with shaking, suggesting that increased yields of FHA could be due to this stabilising effect of CD. Virulent X-mode B. pertussis were maintained in continuous culture. CD caused an increase in culture density in continuous culture indicating that an inhibition of growth was acting in its absence. Continuous culture was not suitable for vaccine production as it did not allow maximum expression of vaccine components and there was a possibility of selection of phase IV organisms. A fermentation process suitable for vaccine production was developed using a stainless steel fermenter and medium containing CD. Antigenic modulation of B. pertussis produced C-mode variants lacking LPF, FHA, fimbriae, X-mode specific outer membrane proteins and adenylate cyclase. C-mode cells had a reduced ability to adhere to Vero cells and reduced hydrophobicity. Antigenic modulation was reversible in continuous culture and the rate of loss of X-mode components was approximately equal to the theoretical washout rate for a number of dilution rates, indicating a common mechanism of control of expression. The addition of CD to C-mode medium partially overcame the repression of X-mode components.