Treatment of F344 rats and B6C3F1 mice with C.I. Pigment Red 3 in the diet (10, 5.0, 2.5, 1.25, 0.6 or 0.3%) for 14 and 90 days resulted in haematological alterations consistent with haemolytic anaemia. Rats appeared to be more sensitive than mice to the haematological effects. Histological lesions were observed in rats and mice after exposure for 90 days. Target organs in the rat were the spleen, bone marrow, liver and kidney. Lesions in the spleen consisted of a haematopoietic cell proliferation, iron-positive pigment and congestion of the red pulp, and inflammation of the splenic capsule. Changes in the livers of rats consisted of haematopoietic cell proliferation and iron-positive pigment in Kupffer cells. Haematopoietic cell proliferation also occurred in the bone marrow of treated rats. The presence of iron-positive pigment and a slightly increased incidence of protein casts were seen in the kidney. Target organs in mice were the spleen, liver and kidney. Histological lesions in mice after exposure for 90 days included increased haematopoietic cell proliferation in the liver and spleen, and iron-positive pigment in the spleen. Mild cytomegaly of the renal tubular epithelia was also observed in exposed mice.