The present study was undertaken to investigate the effect of in vivo administration of neem oilintra-peritoneally (i.p.) to mice bearing a progressively growing transplantable T cell lymphomaof spontaneous origin, designated as Daltons lymphoma (DL), on the tumor growth. Mice wereadministered various doses of neem oil mixed in groundnut oil, which was used as a dilutingvehicle or for administration to control DL-bearing mice. Administration of neem oil resulted inan acceleration of tumor growth along with a reduction in the survival time of the tumor-bearinghost. Neem oil administered DL-bearing mice showed an augmented apoptosis in splenocytes,bone marrow cells and thymocytes along with an inhibition in the anti-tumor functions of tumor-associated macrophages. Thus this study gives an altogether a novel information that neem oilinstead of the popular belief of being anti-tumor and immunoaugmentary may in some tumor-bearing conditions, behave in an opposite way leading to an accelarated tumor progression alongwith a collapse of the host’s anti-tumor machinery. These observations will thus have long lastingclinical significance, suggesting caution in use of neem oil for treatment of cancer.