Systemic anthrax manifests as toxemia, rapidly disseminating septicemia, immune collapse, and death. Virulence factors include the anti-phagocytic gamma-linked poly-d-glutamic acid (PGA) capsule and two binary toxins, complexes of protective antigen (PA) with lethal factor (LF) and edema factor. We report the characterization of LF, PA, and PGA levels during the course of inhalation anthrax in five rhesus macaques. We describe bacteremia, blood differentials, and detection of the PA gene (pagA) by PCR analysis of the blood as confirmation of infection. For four of five animals tested, LF exhibited a triphasic kinetic profile. LF levels (mean +/- standard error [SE] between animals) were low at 24 h postchallenge (0.03 +/- 1.82 ng/ml), increased at 48 h to 39.53 +/- 0.12 ng/ml (phase 1), declined at 72 h to 13.31 +/- 0.24 ng/ml (phase 2), and increased at 96 h (82.78 +/- 2.01 ng/ml) and 120 h (185.12 +/- 5.68 ng/ml; phase 3). The fifth animal had an extended phase 2. PGA levels were triphasic; they were nondetectable at 24 h, increased at 48 h (2,037 +/- 2 ng/ml), declined at 72 h (14 +/- 0.2 ng/ml), and then increased at 96 h (3,401 +/- 8 ng/ml) and 120 h (6,004 +/- 187 ng/ml). Bacteremia was also triphasic: positive at 48 h, negative at 72 h, and positive at euthanasia. Blood neutrophils increased from preexposure (34.4% +/- 0.13%) to 48 h (75.6% +/- 0.08%) and declined at 72 h (62.4% +/- 0.05%). The 72-h declines may establish a "go/no go" turning point in infection, after which systemic bacteremia ensues and the host's condition deteriorates. This study emphasizes the value of LF detection as a tool for early diagnosis of inhalation anthrax before the onset of fulminant systemic infection.