Interferon-gamma-induced GTPases are key to the protective immunity against microbial and viral pathogens. As yet, the cell interior has been regarded as the exclusive residence of these proteins. Here we show that a member of this group, human guanylate binding protein-1 (hGBP-1), is secreted from cells. Secretion occurred in the absence of a leader peptide via a nonclassical, likely ABC transporter-dependent, pathway, was independent of hGBP-1 GTPase activity and isoprenylation, and did not require additional interferon-gamma-induced factors. Interestingly, hGBP-1 was only secreted from endothelial cells but not from any of the nine different cell types tested. Clinically most important was the detection of significantly (P<0.001, Mann-Whitney U-test) increased hGBP-1 concentrations in the cerebrospinal fluid of patients with bacterial meningitis (n=32) as compared to control patients (n=74). In this first report of a secreted GTPase, we demonstrate that secreted hGBP-1 may be a useful surrogate marker for diagnosis of bacterial meningitis.