Tubercle bacilli labelled with C(14) were prepared by growth on radioactive substrates such as glycerol, CO(2), and acetate. These organisms were exposed in vitro to leucocytes (mostly polymorphonuclear leucocytes) from peritoneal exudates of guinea pigs. The respiration of the leucocytes and of the bacilli, alone and together, was followed by determining oxygen uptake and C(14)O(2) production. When heat-killed labelled tubercle bacilli were exposed to leucocytes there was little or no degradation of bacillary material to C(14)O(2) by leucocytic enzymes. On the other hand, conversion of components of sonically disrupted bacilli to C(14)O(2) by leucocytes was significant. It was possible to determine the oxygen uptake and C(14)O(2) production of phagocytized living tubercle bacilli, and it was found that after phagocytosis the bacilli maintained their rates of oxygen consumption and C(14)O(2) production. This finding was in contrast to observations made with Mycobacterium phlei, a saprophytic acid-fast organism, and with Bacillus subtilis. In these cases oxygen consumption and C(14)O(2) production declined after phagocytosis, and bacterial components were converted to carbon dioxide to a significant degree by leucocytic enzymes.