During the past twenty years 793 strains of Listeria monocytogenes were isolated from human beings in the Netherlands; of these 193 were taken from neonates and babies up to two months old. Isolations from adults came from patients (242) as well as clinically healthy people (358). Septicaemia appeared to occur to an equal extent in men and women (20 and 29 respectively), whereas meningitis was found more often in men than in women (50 and 22 respectively). A clear predisposition for L.m. develops after administration of immunosuppressive treatment and also in cases of liver disorders. Among veterinary surgeons listeriosis has been observed as an occupational disease. Isolations from animals have shown that L.m. may cause infections in every species of warm-blooded animal. Next to meningitis and abortus chronic and atypcial symptoms of the disease may be observed in animals. Epizootic spread of the disease has hardly ever been observed in the Netherlands. In clinically healthy humans and animals both the haemolytic and the non-haemolytic type of L.m. have been isolated from feces; as regards the latter type it is very questionable whether it has any pathogenic significance. When inoculating 10 days old hen's eggs the haemolytic strains will kill all embryo's within 4 days whereas all embryo's inoculated with non-haemolytic strains will survive. Both types of strains have also been isolated from waste- and surface waters. As regards epidemiological and epizootiological aspects the conclusion is warranted that continued research will be needed to get a clear picture of the course of infection of L.m. When isolating strains from contaminated material the so-called cold enrichment icubation at 4 degrees C continues to be of great value; in the course of our experiments the nalidixic-acid trypaflavine serum agar proved to be a very good selective medium. A number of stable biochemical reactions of L.m. are rather characteristic (salicine+, galactose-) but provide no clue to distinguish between haemolytic and non-haemolytic strains.