We have developed a rapid, accurate, and inexpensive hemagglutination assay (HA) for diagnosis of Indian visceral leishmaniasis (VL) using achatininH, a 9-O-acetylated sialic acid binding lectin. The assay is based on the selective binding of achatininH to 9-O-acetylated derivatives of sialic acid (9-O-AcSA), which appears exclusively on the erythrocytes of patients with VL. Blood samples from 35 patients with demonstrable splenic or bone marrow amastigotes were assayed by HA and compared with an ELISA. All cases were correctly diagnosed by HA, and were in concordance with the ELISA. The increase in 9-O-AcSA was corroborated by direct fluorimetric estimation of sialic acid and its 9-O-acetylated derivative. Therefore, 9-O-acetylated derivatives of sialic acid, absent in normal erythrocytes, serve as an important biomarker, and indirectly reflect the presence of the parasite. Accordingly, this assay can be used for detection of active kala-azar cases and merits clinical consideration.