Human monocytes maintained in culture lose microbicidal activity against intracellular protozoa which has been correlated with attenuation of the respiratory burst. The granule enzyme myeloperoxidase, which can markedly amplify hydrogen peroxide-pendent antimicrobial activity, is also lost in vitro. Adherent monocytes were examined immediately, 3 and 10–14 days following explanation, for the magnitude of the stimulated respiratory burst and for cellular myeloperoxidase. Fresh cells generated 254 ± 38 nmol O2-/mg protein as compared to a peak of 782 ± 45 nmol O2-/mg at 3 days and < 100 nmol O2-/mg after 10–14 days. The myeloperoxidase content of the cells also decreased; over 85% was lost after 3 days. Fresh monocytes killed over 90% of ingested Toxoplasma gondii or Leishmania major. In contrast, 10–14 day explanted monocytes killed only 12% of ingested Toxoplasma and 33% of Leishmania, and surviving organisms replicated readily. The 3-day monocytes were significantly less able to kill protozoa than were fresh cells despite their nearly 3-fold greater generation of O2-. If peroxidase was reintroduced into 3-day monocytes by coating organisms with eosinophil peroxidase prior to phagocytosis, their antiprotozoa activity was nearly restored to that of freshly explanted cells.