A positive correlation exists between the pathogenicity of bacteria and fungi when evaluated in the insect Galleria mellonella and mice. This work sought to determine whether fluctuations in the number of haemocytes and the proliferation of yeast cells in infected larvae could be used to determine the relative pathogenicity of a range of yeast isolates. Larvae were inoculated with 1 x 10(6) stationary-phase yeast cells and incubated in the dark at 30 degrees C for 48 h. The results indicated that larvae inoculated with the most pathogenic isolates (i.e. those capable of killing >80% of infected larvae) showed a significant reduction in haemocyte density. Larvae inoculated with isolates of low pathogenicity (i.e. capable of killing <20% of infected larvae) demonstrated only a small fluctuation in haemocyte numbers. The most pathogenic yeast isolates proliferated in the larvae, whereas the isolates of low pathogenicity did not. These results demonstrate a relationship between the ability of yeast isolates to kill larvae and changes in haemocyte density and yeast cell density in infected larvae. These end points may extend the applicability of the G. mellonella system for use with a wider range of microbial isolates.