A rigorous mathematical model is developed to describe the distribution of respiration-generated oxidants among reactive sites within the phagolysosomes of leukocytic cells. Reaction parameters include the diffusion coefficient of the oxidant, the intrinsic rate constants for its reaction with the phagosomal membrane and the cell envelopes of entrapped bacteria, the overall rate constant for its reaction with solution components of the phagosomal fluid, and the phagosomal dimensions. The model is used to describe the dynamics of randomly generated .OH and HCO3. radicals within the phagosome. These radicals were chosen because the necessary rate parameters either have been measured or could be reasonably estimated. The calculations show that .OH radical cannot be an effective bactericide unless generated in the immediate vicinity of the bacterial surface because its extreme reactivity precludes any significant diffusion. The HCO3. radical, however, is predicted to be a very effective toxin, even when relatively high concentrations of oxidant scavengers are present in the phagosomal fluid. In the absence of complicating features, the reactivity patterns of other less reactive oxidants (e.g., metal oxo or peroxo complex ions) are predicted to be very similar, although quantitative analysis is precluded by the lack of relevant rate data. For these oxidants, the predicted intraphagosomal toxicities differ markedly from toxicities measured in dilute bacterial suspensions because differences in mean diffusion lengths of the oxidants are unimportant in environments with the dimensions of phagosomes, but very important under in vitro conditions. The model is general and can be applied to other reactions occurring in similar microheterogeneous cellular and subcellular environments.