A sialidase (neuraminidase, acylneuraminosyl hydrolase, EC 3.2.1.18) has been discovered and isolated from Gardnerella vaginalis (ex. Haemophilus vaginalis), a possibly pathogenic inhabitant of the female genital tract. Bacteria were grown in peptone-yeast-extract medium with 2.0 mM N-acetylmannosamine as enzyme inductor under CO2 atmosphere. Sialidase activity was found in the bacterial sediment and in the culture medium. The enzyme was liberated from the cells by ultrasonic treatment. Purification was performed by 60-80% ammonium sulfate precipitation and by column chromatography on Sepharose CL-6B and Sephadex G 200. The enzyme revealed a molecular weight in the range of Mr 75 000 and a pH optimum at 5.5. Among the different types of NeuAc-containing glycoconjugates, the enzyme exhibits its highest activities towards the globular glycoproteins alpha 1-acid glycoprotein and fetuin. Taking their cleavage rate as 100, it is around 55 for II3NeuAc-Lac, 45 for bovine submaxillary mucin, 35 for II6NeuAc-Lac and IV3, III6NeuAc2-LcOse4. The rates for III8,II3NeuAc2-Lac, gangliosides and colominic acid are below 20. Due to its specificity pattern, the enzyme may play a role in the pathogenic process of G. vaginalis infections.