Porphyromonas gingivalis strain W50 was grown under haemin-limitation and haemin-excess conditions in a chemostat at pH 7.5. The maximum specific growth rate (mumax) was determined at both haemin concentrations (mumax = 0.236 +/- 0.052 and 0.271 +/- 0.039 h-1, respectively). This enabled dilution rates to be adjusted so that the virulence and enzyme activity of haemin-limited and haemin-replete cells could be compared at identical relative growth rates (murel) of 0.25, 0.50 and 0.75 of their respective mumax. The data showed that the fastest growing cells were significantly more virulent than those grown more slowly, irrespective of haemin concentration. However, at each growth rate tested, cells grown under haemin-excess conditions were always more virulent than haemin-limited cells. Trypsin-like enzyme activity of whole cultures was also greater at each growth rate under haemin-excess conditions while, conversely, collagenolytic activity was generally higher in haemin-limited cultures. Thus, although growth rate had an effect on the virulence and enzyme activity of P. gingivalis, the availability of haemin for growth was the most significant factor.