The aim of this study was to examine the tumorigenicity of the food colouring phloxine (Food Red 104) in (C57BL/6N x C3H/HeN) F1 mice. Groups of male or female mice received 0, 0.1 and 0.4% phloxine in the diet starting at 6 wk of age for a maximum of 90 wk. Survivals of mice were more than 85% at 64 wk after the start of phloxine administration. Their body weights reached maximum at 48 wk and both male and female mice given 0.1 and 0.4% phloxine weighed significantly more than their respective controls. Tumours developed in 13 organs, the greatest incidence in males being of liver tumours (67.3%), followed by lung tumours (10.9%), lymphomas (9.7%), altered foci of liver (8.4%) and hepatic haemangioma (7.8%). In females, hepatic tumours (11.5%) had the greatest incidence followed by lymphomas (10.8%), pulmonary tumours (8.2%) and hepatic haemangioma (3.2%). In males, the incidence of altered foci and haemangioma in the liver was 19 and 14%, respectively, in the control groups, whereas only 2% occurred in the group given 0.4% phloxine (P < 0.05). It is concluded that average body weights increased significantly in female mice of groups given 0.1 or 0.4% phloxine, compared with body weights of the female controls, and that altered liver foci were significantly reduced in male mice given 0.4% phloxine. There was a significant increase in pituitary tumour incidence in the test groups compared with the control groups.