In 8 volunteers blood and urine bismuth levels were detected after a 5-day course of tripotassium dicitrato-bismuthate and rose further with increasing dose. In 24 patients, blood and urine bismuth were measured on the last day of treatment. There was no significant difference (p greater than 0.2) in either blood or urine bismuth levels amongst patients treated for 4,5 or 6 weeks with colloidal bismuth. None of the blood level measurements exceeded 50 micrograms/l. Bismuth was slowly eliminated in the urine after stopping treatment.