Objective: In the present study, Lactobacillus casei (L. casei) labelled with 15 N was used to follow the metabolic fate of orally administered heat-killed 15 N-labelled L. casei ( 15 N-L.casei) in humans. Design: Experimental study. Setting: University of Rostock, Children's Hospital, Research Laboratory. Subjects: Twelve healthy adults aged 23-32 years. Intervention: The subjects received 36 mg/kg body weight heat-killed 15 N-L.casei and 500 mg Lactose-[ 13 C]ureide together with breakfast. Expired air samples were taken over 14 h, whereas urine and faeces were collected over 2 days. A blood sample was taken after 2 h. 13 C-and 15 N-enrichments were measured by isotope ratio mass spectrometry (SerCon, UK). Results: The orocaecal transit time (OCTT) was reached after 4.1 h. The urinary 15 N-excretion was 9.3% of the ingested dose, whereas the faecal excretion was 65.1% of the ingested dose. After 2 h, 15 N-enrichment of supernatant, fibrinogen, and plasma protein precipitate amounted to 254, 11, and 2 p.p.m. excess, respectively. Conclusions: In comparison to the OCTT of 4.1 h, 15 N-enrichment in urinary ammonia and urinary total nitrogen already began to rise 30 min after 15 N-L.casei ingestion, indicating that 15 N-L.casei is rapidly digested in the small bowel. This is confirmed by 15 N-enrichments of blood plasma fractions. The ingestion of heat-killed 15 N-L.casei led to a total excretion of 74.4% of the ingested 15 N-dose.