Twenty-six patients with rheumatoid arthritis which was poorly controlled despite high dose D-penicillamine were studied. Compliance was assessed by standard methods (return tablet count and interview). In addition low-dose phenobarbitone was included in the penicillamine formulation as a pharmacological indicator of compliance. Using these techniques incomplete compliance was apparent in 11 patients (42%). All such patients were identified by the pharmacological marker. Only one admitted poor compliance at interview and only six returned more than a few tablets too many. The reason for the high incidence of poor compliance in this selected group is not apparent but it may represent a significant cause of failure with D-penicillamine therapy. The use of low-dose phenobarbitone may have wider applications in the investigation of patients with other conditions who fail to respond adequately to treatment.