In the baboon or the mouse, a stimulus such as LPS, TNF, or IL-1 typically led to a rapid induction of circulating IL-6, the levels peaked by 2 to 3 h and then declined to near-baseline values by 6 to 8 h. Administration to baboons or mice of "neutralizing" anti-IL-6 mAb followed by an IL-6 inducer led to a marked and sustained increase in circulating IL-6 levels. IL-6 Ag, IL-6 biologic activity, neutralizing anti-IL-6 mAb, and IL-6/anti-IL-6-mAb complexes could all be observed for an extended period of time (beyond 8 h) in the circulation of such animals. Nevertheless, in mice, if the anti-IL-6 mAb had been administered before the IL-6 inducer, there was a reduction in the in vivo IL-6-induced stimulation of fibrinogen levels, indicating that most of the intravascular IL-6 was not readily available for eliciting hepatocyte effects under these experimental conditions. Intraperitoneal administration into mice of mixtures of murine rIL-6 or human rIL-6 together with their respective anti-IL-6 mAb led to a marked increase in the appearance and longevity in the peripheral circulation of the exogenously administered murine or human rIL-6 species in a biologically active form. Varying the ratio of human rIL-6 to anti-human IL-6 mAb indicated that a molar ratio of 1:1 was sufficient for the ability of mAb to chaperone IL-6 in the murine circulation. Human rIL-6 mixed with "neutralizing" mAb in the approximate ratio 1:1 elicited an enhanced fibrinogen response in vivo in the mouse; an IL-6:mAb ratio of 1:125 led to a reduction in the fibrinogen response even though the levels of circulating B9 bioactivity and of human rIL-6-Ag were maximal under these conditions. Gel-filtration chromatographic and Western blotting analyses of IL-6 present in vivo in the mAb-free baboon revealed that although the IL-6 Ag was largely present in high molecular mass complexes of size 400 kDa in association with soluble IL-6 receptor, the B9 bioactivity was largely of low molecular mass (20 kDa). In contrast, in the anti-IL-6 mAb-treated baboon or mouse, the IL-6 Ag and bioactivity were both largely in complexes of 200 kDa. Thus, the binding of IL-6 in the intravascular compartment to other proteins, anti-IL-6 mAb in the present studies, gives IL-6 unexpected biochemical and pharmacologic properties in vivo.