Propionic acid at all levels tried (0.125, 0.25, 0.5, 1.0 and 2.0 mg/g of substrate) decreased the total mold count of poultry feed of 15 or 20% H2O content, but increased the count at 25 or 30% H2O content. The paradoxical effect which was time-dependent could be explained on the basis that the total mold count reflected the status of spores rather than the total fungal activity. Assessments of antifungal activity based on respiratory activity, such as weight loss of substrate, heat production and changes in CO2, O2, and H2O content of feed but not pH, did not display such paradoxical behavior. Measurement of CO2 production from feed and ingredients for assessment of antifungal agents was preferred for its ease, speed, economy, accuracy and precision. The findings that most of the CO2 was produced aerobically (assuming a respiratory quotient of 1.0), that yeasts, bacteria, and some fungi produced some CO2 anaerobically, that selective inhibitors of isolates from com meal could not differentiate the production of CO2 from corn meal by bacteria, yeast and fungi, that all three types of microorganisms were active at water concentrations likely to occur in stored feed and ingredients, and that differential plate counts revealed increases in all three classes suggested that all three classes are involved in deterioration of feed ingredients. Consequently, antifungal agents used in feed and ingredients need broad antimicrobial activity and the methods for the assessment of antifungal agents should detect this broad spectrum of activity.