Current laboratory test methods to measure efficacy of new mold inhibitors are time consuming, some require specialized test equipment and ratings are subjective. Rapid, simple quantitative assays to measure the efficacy of mold inhibitors are needed. A quantitative, colorimetric microassay was developed using XTT tetrazolium salt to metabolically assess mold spore germination. Quantitative values for the inhibition of mold spore germination using the XTT assay were 30-fold lower for thiabendazole and Durazol than the calculated MIC90 based on subjective ratings using current standard methods. The XTT assay can be used to demonstrate the relative resistance of different fungal species to antifungal agents, to compare the relative inhibition of a new antifungal agent to existing antifungal agents, or to screen new wood preservative formulations for their susceptibility to mold growth.