On the question of a carcinogenic action of cobalt-containing compounds.

After subcutaneous injections of doses of 5 x 2 and 1 x 10 mg cobalt (II) oxide/kg/week (total dose 1,000 mg/kg), respectively 5 out of 10 and 4 out of 10 rats in a lifetime study developed local malignant tumours (controls 0/10). After intraperitoneal injections of 3 x 200 mg cobalt (II) oxide/kg 14 out of 20 rats developed malignant intraperitoneal tumours (controls 1/20). With a Co/Al/Cr spinel the same treatment produced malignant intraperitoneal tumours in only 2 out of 20 rats. After intratracheal instillation of cobalt (II) oxide in single doses of 2 mg/kg (total dose 78 mg/kg) and 10 mg/kg (total dose 390 mg/kg) there were 2 benign pulmonary tumours among 100 rats in the low-dose group and 2 benign and 4 malignant pulmonary tumours among 100 rats in among 100 rats in the high-dose group. The Co/Al/Cr spinel was tested intratracheally only in the high dose (total dose 390 mg/kg); among the 100 rats this produced 3 malignant pulmonary tumours. Alternating intratracheal instillation of cobalt (II) oxide (total dose 470 mg/kg) and benzo[a]pyrene (total dose 200 mg/kg) led to 9 malignant pulmonary tumours in 20 rats, whereas benzo[a]pyrene alone caused only 1 malignant pulmonary tumour in 20 rats. The results suggest that cobalt (II) oxide (76.7% Co) has a weakly carcinogenic effect. The Co/Al/Cr spinel investigated is still less active, and even in very sensitive tests (i.p. and i.t. administration) shows no statistically significant carcinogenic effect. This smaller effect may possibly be explained by its lower cobalt content (24.0%) or its much lower solubility (only less than 10% of the solubility of CoO in 0.1 n HCl).