Although there is considerable interest in identifying mercury-resistant bacteria, no standardized assay exists for this purpose. In this study, the effect of the composition of the medium on the susceptibility of oral streptococci to HgCl2 Was investigated. The minimum inhibitory concentration (MIC) of HgCl2 for 52 streptococcal strains and the reproducibility of MIC values for Hg-sensitive and Hg-resistant strains was determined with 11 different media. Addition of blood increased the MIC values, and some media (tryptone soya agar, with or Without blood) could not discriminate between Hg-sensitive and Hg-resistant strains. The proportion of streptococci that appeared to be resistant to Hg was very high (>70%) on some media (mitis-salivarius, tryptone soya, Columbia), but not on others (Mueller-Hinton, Bra-in Heart Infusion, Isosensitest). The MICs of the control strains varied considerably on different testing occasions for tryptone soya agar (with and without blood), Isosensitest agar, and Columbia agar (with blood). Mueller-Hinton (without blood) appeared to be the most suitable medium for isolating Hg-resistant oral streptococci.