The results of disk diffusion and plate dilution susceptibility testing of Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, and Proteus mirabilis on media gelled with agar-agar or with a synthetic hydrogel were compared. Synthetic hydrogel can be combined with a totally defined synthetic amino acid medium to yield a reproducible, totally defined, synthetic solid medium without the antagonistic or booster effects of some components of agar. Such a medium could be used as a reference medium for susceptibility testing.