BACKGROUND An enhanced presence of 9-O-acetylated sialoglycoconjugates (9-O-AcSGs) triggers the alternate pathway (AP) in Indian visceral leishmaniasis (VL). Antibodies directed against these epitopes are present in high titers. The biological relevance of these antibodies, with regard to activation of the classical pathway (CP), was investigated. METHODS Complement activators were affinity purified, complement activation via the CP, AP, and lectin-mediated complement pathway was measured by use of an anti-C3 radio-binding assay, and the number of C3 molecules was quantitated by Scatchard analysis. Cell death induced via the complement pathways was measured by use of MTT (tetrazolium salt 3- [4, 5-dimethylthiazol-2-yl] -2, 5-diphenyltetrazolium bromide) assay, and uptake of propidium iodide (PI) was measured by flow cytometry. RESULTS Anti-O-AcSGs from both healthy donors and patients with VL elicited C3 deposition as early as 3 min, which triggered parasite lysis, as demonstrated by use of MTT assay and corroborated by the high rate of uptake of PI. Analysis of complement activation by mannan-binding lectin and C-reactive protein demonstrated their negligible contribution during the 3-min time frame. CONCLUSIONS Anti-O-AcSGs were identified as an important source of CP activation under normal physiological conditions, suggesting that they play a role in conferring host protection against parasite infection.