Objective: In this study, heat-killed Lactobacillus johnsonii (La1), doubly labelled with 13 C and 15 N (hk-dlLa1), was used to follow the metabolic fate after oral administration in humans. Design: Experimental study. Setting: University of Rostock, Children's Hospital, Research Laboratory. Subjects: Ten healthy adults aged 23-26 years. Intervention: The subjects received 74.6 mg/kg body weight hk-dlLa1 and 10 g a-D-raffinose together with breakfast. A sample of venous blood was taken after 2 h. Expired air samples were taken over 14 h, whereas urine and faeces were collected over a period of 48 h.13 C-and 15 N-enrichments were measured by isotope ratio mass spectrometry. Hydrogen concentrations were measured by electrochemical detection. Results: The orocaecal transit time (OCTT) was reached after 3.4 h. After 2 h, 13 C-and 15 N-enrichment of fibrinogen amounted to 2 and 25 p.p.m. excess, respectively. The 13 CO 2 -exhalation amounted to 9.2% of the ingested dose. The urinary excretion of 13 C and 15 N was 2.1 and 10.4% of the ingested dose, respectively, whereas the faecal excretion was 47.9 and 43.7% of the ingested dose, respectively. Conclusions: In comparison to OCTT of 3.4 h, both stable isotopes appear after 30 min in breath and urine, indicating that hk-dlLa1 is rapidly digested in the small bowel before reaching the caecum. This is confirmed by 13 C-and 15 N-enrichments of blood plasma fractions. The ingestion of hk-dlLa1 led to a 13 C-and 15 N-excretion of 59.2 and 54.1% of the ingested dose, respectively, of both stable isotopes.