In order to elucidate the ecological role of bacteriophages in the human intestine, we analysed the numbers of coliphages and of coliphage strains present in faecal samples collected from healthy individuals and from patients with certain intestinal diseases. The isolated phages were grouped according to their serological properties. The samples with low phage titres, observed in both healthy subjects and patients, contained mainly temperate phages (many were related to phi 80 and lambda), and those with higher titres, observed in patients, contained virulent phages. From successive surveys of coliphages and their host, Escherichia coli, in faecal samples of each subject, it was concluded that temperate phages are maintained in the human intestine through spontaneous induction of lysogenic bacteria. Qualitative and quantitative differences existed between phages isolated from faecal samples from healthy subjects and from patients. Simultaneous changes in the distribution patterns of coliphages and of the clinical symptoms were observed in a continuous survey of a leukaemic patient in a protective environmental ward.