Plasma samples of seven patients with gut and pancreatic endocrine tumours who have been on long-term treatment with a long-acting somatostatin analogue (SMS 201-995) were investigated for endogenous antibodies to the peptide by incubation with radiolabelled SMS 201-995. The duration of treatment with the somatostatin analogue was between 9 and 26 months and the dose from 100 to 300 micrograms day-1. In none of the patients could antibodies to SMS be detected. The effect of this somatostatin analogue is unlikely to be impaired by formation of endogenous antibodies, even after long-term treatment.