Groups of 50 male and 50 female B6C3F1 mice were given magnesium chloride (MgCl2.6H2O) at dose levels of 0 (control), 0.5 and 2% in the diet for 96 wk, after which all animals received the control diet for 8 wk and were then necropsied. In females of the high-dose group a decrease in body weight was observed. However, survival rates did not differ between the treatment and control groups for males or females. Furthermore, clinical signs and urinary, haematological or serum clinical chemistry parameters showed no treatment-related effects. On histological examination, tumours were mainly found in the skin/subcutis, liver and lymphatic system. However, with the exception of a significant decrease in the incidence of liver tumours among males of the high-dose group, no differences were noted in the tumour incidence between the treated and control animals. Thus, the study described here clearly shows a lack of carcinogenicity of MgCl2.6H2O given to B6C3F1 mice in the diet.