Male and female mice were administered (gastric intubation) 24.8×108conidia/mouse, the equivalent of a 70-kg man receiving a 0.4-ha application of conidia. No deleterious effects were observed and greater than 99.9% of the infectivity of conidia was lost during passage through the alimentary tract. Also,>99% of the total infectivity that was recovered from feces was recovered one day after feeding conidia. No apparent clinical, pathological, or histological abnormalities were observed when white rats were exposed to 1.1 × 107viable conidia/liter of air for one h, the equivalent of a 70-kg man inhaling conidia applied to ca. 1/500 of a ha. Eyes and the abraded skin of rabbits treated with a dose of 1.2×108conidia/eye or 3×109conidia/cm2of skin, respectively, showed no apparent clinical, pathological, or histological abnormalities. These studies demonstrated thatN. rileyishould not pose a serious, acute hazard to mammals.