This work aims at investigating the anticlastogenic activity of the drug silymarin. To achieve such a purpose four doses of the drug i.e., 2, 4, 8, and 20 mg/ kg. b. wt. were tested employing mice genome (Mus musculus, 2n= 40).                 The following genotoxic bioassays were used: 1- Estimation of cell proliferation. 2- Analysis of chromosomal abnormalities in mice bone marrow cells. 3- In vivo induction of sister chromatid exchanges, 4- Micronucleus test, and 5- Analysis of primary spermatocytes. Cyclophosphamide (Indoxan) was used as a positive control.                 The results obtained revealed that silymarin was proven to be capable in causing significant increases in cell proliferation (estimated as mitotic index) of mice bone marrow cells; decreasing in total aberrant metaphases as well as in SCEs in vivo.                 Micronucleated polychromatic erythrocytes and aberrant diakinesis were significantly decreased; giving an evidence that silymarin has a strong anticlastogenic activity upon mice genome in somatic as well as germinal cells.