ATP provides a biologically significant tool for quantitating either functional biomass or growth potential. Ultrasensitive detection of ATP plus a novel extraction of microbial ATP have decreased the hitherto required number of microbial cells per assay 60 fold. A 400x safety factor eliminates host ATP. Thus, we are in a position to investigate the energetics of host-grown and host-dependent microbes. The objectives are: 1. To define approaches and conditions for the in vitro cultivation of M. leprae by pursuing current work in which the noncultivated Mycobacterium lepraemurium (agent of rat leprosy) is used as an interim model for M. leprae. ATP determinations have substantiated reports that M. lepraemurium is able to grow microscopically in diffusion chambers devoid of host cells, provided the chambers were incubated in the peritoneal cavities of mice. By means of ATP, the leaching rates of the "in vivo-type cell membranes" in M. lepraemurium have been characterized, then compensated during long term refrigeration in membrane stabilizers plus cofactors and metabolites from other microbes. It is proposed to continue these investigations until cultivation of M. lepraemurium in vitro has defined conditions to be regulated during work on the less competent M. leprae. 2. To quantitate the ATP in Mycobacterium leprae in terms of ATP per aliquot (biomass) and ATP per counted cell (growth potential) during the progression, regression and immuno- or chemo-therapy of leprosy. ATP data will also be employed to select optimal sources of M. leprae for laboratory investigations and to monitor the preservation of stock suspensions and stock strains by freezing or by equilibrating metabolic pools during refrigeration.