Female laboratory bred albino mice (2n = 40) were orally administered cesium chloride (CsCl) in aqueous solution as a single dose and the damage induced at the chromosomal level was observed in bone marrow cells after 6, 12, 18, and 24 hours of exposure. The concentrations of the chemical given were calculated as fractions of the LD50, namely 1/5, 1/10, and 1/20. The cytogenetic endpoints screened for were chromosomal aberrations (CA) and divisional frequency or mitotic index (MI). The frequency of chromosomal aberrations induced was directly proportional to the concentration of the chemical administered. The highest dose was the most toxic and was considered to be the maximum tolerance level. Effects on divisional frequency were variable, the highest concentration being significantly mitostatic, the middle one ineffective, and the lowest slightly mitogenic. In general, the observations indicate that CsCl is clastogenic when administered orally to mice in vivo and the effects are dose-dependent.