Phosphine is an important fumigant in the grain industry and has been reported to be genotoxic in occupationally exposed fumigators. This study reports on the effects of phosphine inhalation exposure at up to, and exceeding, occupational relevant levels in a subchronic (0.3, 1.0 and 4.5 ppm, 13 weeks) and a short term repeated-dose (5.5 ppm, 2 weeks) study in both sexes of Balb-c mice. The following end-points were examined: micronucleus induction in bone marrow, peripheral blood, spleen lymphocytes and skin keratinocytes, mutations at the hypoxanthine-guanine phosphoribosyl transferase locus in lymphocytes, and weight gain and relative organ weights (kidneys, lungs, liver, heart, brain and spleen). After subchronic exposure, there was a highly significant negative linear correlation between proportional weight gain and exposure in both sexes (multiple linear regression, r = -0.56, P < 0.0001), with female mice showing a greater effect. Females also showed an increase in relative organ weights at the highest test dose, in contrast to males where there was a slight decrease. A statistically significant increase in micronucleus frequency was seen in the bone marrow and spleen lymphocytes of both sexes, but only at the highest concentration. The short term repeated-dose study revealed a slight decrease in weight gain in both sexes, with a greater effect in females. It is concluded that phosphine is weakly genotoxic in both sexes of mice, and has an effect on weight gain. However, the weak genotoxic effect may not be biologically significant as it was seen only in the subchronic study and only at the highest test concentration of 4.5 +/- 0.8 ppm (approaching the LD50). Although such exposure conditions are unlikely to be encountered in an occupational environment, caution should continue to be exercised in the use of phosphine until more data become available.