Trichomonas vaginalis possesses a membrane-associated neuraminidase activity that is released into culture medium during its growth in vitro. The neuraminidase shows an optimum pH of 4.5 and a Km of 0.15 mM for 2'-(4-methylumbelliferyl)-alpha-D-N-acetyl-neuraminic acid as a substrate. This enzyme releases mainly alpha-2,3-linked sialic acid because it is able to liberate sialic acid from sialyllactose (mainly alpha-2,3) but not from mucin (alpha-2,6) or fixed erythrocytes (mainly alpha-2,6). The neuraminidase activity is strongly inhibited by 2,3-dehydro-2-deoxy-N-acetyl neuraminic acid, whereas EGTA and Ca2+ do not affect the activity. Gel filtration-fast protein liquid chromatography of culture supernatant displays a single peak of neuraminidase activity with molecular weight 52,000. The levels of neuraminidase activity are variable in fresh and long-term grown isolates of T. vaginalis, regardless of time in culture. However, there are 2 kinds of isolates, 1 group with high neuraminidase activity and able to secrete the enzyme during growth and the other with low neuraminidase activity. The results suggest that T. vaginalis possesses a membrane-associated neuraminidase that is present to a variable degree in different isolates.