Three cancer cell lines, i.e. HEpG-2 (human liver carcinoma), MCF-7 (human breast adeno-carcinoma), and PC3 (Prostatic Small Cell Carcinoma) were used to determine the cytotoxic effects of the neonicotinoid insecticide (imidacloprid) and conazole fungicide (myclobutanil). Cytotoxicity was measured by neutral red incorporation (NRI) assay. The lowest concentration of the tested pesticides (0.5 μg/ml) was toxic. With the increase of the concentration up to 80 μg/ml, the Department of plant protection, Faculty of Agric., Ain Shams University, shoubra  Elkheima, Cairo, Egypt Department of Genetics, Faculty of Agric., Ain Shams University, Cairo, Egypt damage degree of the cellular form and size was more serious. The midpoint cytotoxicity value, (NRI50) for imidacloprid and myclobutanil for HEpG-2, MCF-7, and PC3 cancer cell lines were 110.5, 67.7 and 67.6 μg/ml and 38.12, 41 and 27.5 μg/ml, respectively. In general, myclobutanil was very toxic in the three cancer cell lines compared with imidacloprid.