First, now that germfree animals have been reared for generations and on a larger scale, we need current information about the general characteristics of the germfree stock. More specifically, we must ask ourselves whether various germfree species, as they are available today, truly represent the standardized biological tool of our long-standing anticipation. A second reason, stressing the need for descriptive work on the germfree animal, concerns its broad use in the field of experimentation. The germfree host, differing from normal stock not only by its absence of the bacterial flora but also by its lack of systemic responses to that flora, is an animal that is considerably different from the conventional one. The average worker who wishes to use the germfree tool occasionally cannot possibly gauge for himself the details of this broader background, yet it is clear that these details will affect his results. Therefore, it appears that there is need for the elaboration of a limited but comprehensive parameter in this field. A third justification for descriptive work in the germfree animal concerns the environmental physiologist. A study of the germfree host in comparison to the normal stock, including animals exposed to controlled bacterial contamination, portrays the effects of the normal flora on the animal with great clarity.