A neem leaf preparation (NLP) was investigated for its role in the induction of tumor cell apoptosis to elucidate the mechanism of NLP mediated immunoprophylaxis in tumor growth restriction. As NLP did not induce direct apoptosis of human tumor cell lines KB, MCF7 and K562, it was used instead to stimulate human peripheral blood mononuclear cells (PBMC) for 72 h. The PBMC derived culture supernatant (NLP-CS) was observed to induce the restriction of tumor cell proliferation as well as apoptosis. An enzyme linked immunosorbant assay revealed the presence of cytotoxic cytokines, IFN-gamma and TNF-alpha, in the NLP-CS. The inhibition of secretion of IFN-gamma and TNF-alpha in NLP-CS caused a significant decrease in tumor cell apoptosis. Furthermore, stimulation of these tumor cells with NLP-CS resulted in upregulation of the caspase 3 and downregulation of the Bcl 2 and cyclin D1. These observations suggested that NLP could induce tumor cellular apoptosis by releasing cytotoxic cytokines from human PBMC.