The hemolymph-derived achatinin_h (lectin) from Achatina fulica showed a marked cytotoxic effect on MCF7, a human mammary carcinoma cell line. IC₅₀ values as measured by the 3-(4,5-dimethylthiazol-2yl)-2,5-diphe-nyltetrazolium bromide assay for achatinin_h ranged from 6 to 10 μg/ml in the MCF7 cells. MCF7 cells showed significant morphological changes leading to cell death. The above cell death was observed after 48 h of treatment with 8 μg/ml when compared to untreated cells. Alterations in the tumor marker enzymes, as well as in antioxidant enzymes, were observed after achatinin_h treatment. The specificity and purity of the achatinin_h was confirmed by the Western blot assay. Achatinin_h binding to MCF7 cells was detected by anti-achatinin_h, and visualization of the achatinin_h binding sites on confluent MCF7 cells was confirmed by flourescein isothiocyanate conjugated secondary antibody. MCF7-treated cells fluoresced, indicating the presence of achatinin_h binding sites. Fluorescence-activated cell sorting analysis of the cell cycle showed a significant increase in S-phase in MCF7 cells after 48 h of achatinin_h treatment. The cells were arrested in G₂/M phase of the cell cycle after 48 h with significant changes in cell viability. Cellular damage was confirmed by agarose gel electrophoresis with the characteristic appearance of a DNA streak in treated MCF7 cells indicating the ongoing apoptosis.