Human leptospirosis was diagnosed in 3 patients from the Detroit General Hospital. Epidemiological studies of these cases revealed heavy rat infestations in the residences of 2 patients. Leptospira serotype icterohaemorrhagiae was isolated from the rats and 1 patient. Cultural isolation was attempted from numerous blood and urine samples from each patient. Media containing different selective contaminant inhibitor substances was also utilized. Only 2 samples were culturally positive. It appears that very few viable cells are shed by human patients intermittently. Clinical reports and leptospirosis diagnosis are reported for each case. The agglutinin-absorption test was helpful in elucidating the true etiologic agent in the absence of cultural isolation. A serological survey for leptospirosis was conducted among swine slaughterhouse workers in Detroit. Significant titers to L. pomona were observed.