A survey of the pathology of 567 laboratory-bred cotton-eared marmosets (Callithrix jacchus) is presented. There were few significant pathological changes in animals used in studies up to 6 months in duration, suggesting that the marmoset can be a useful non-human primate species for routine toxicology. The most common pathological changes encountered were chronic colitis, chronic thyroiditis and interstitial mononuclear infiltration in the kidney. No internal parasites were encountered, nor were any viral or bacterial diseases identified. Fungal disease was confined to a few cases of oesophageal mycoses. In a long term study a variety of pathological changes have been observed, including a 'wasting' syndrome, not related to skeletal muscle myopathy, with atrophy of the gastrointestinal tract, salivary glands and gonads, haemosiderosis and fatty change in the liver and osteoporosis. It is suggested that these changes may be related to protein deficiency and that the nutritional requirements of the marmoset require further investigation.