a) 1069 human blood samples were tested for the presence of C. F. antibodies to leptospirae. 98 of these sera were from patients suffering from Weil's syndrome. 13 out of 98 patients were found to be suffering from leptospirosis; one of these died. 460 sera were from patients with febrile (acute) illness of unknown origin and 511 sera from patients with respiratory disease (especially "influenza like disease"). None of these patients was found to be suffering from leptospirosis. b) 191 human sera from healthy people at risk to leptospiral infection were examined by microscopic agglutination test (MAT). A high percentage (14.1%) was found to have antibodies against one or two leptospiral serogroups. The results obtained suggest that the incidence of leptospiral infections in humans in Northern Greece is higher than previously reported and the predominant serogroups were found to be Icterohaemorrhagiae followed by Autumnalis and Canicola.