The excretory–secretory antigen of Fasciola hepatica (ESA) is involved in the suppressive phenomena of cellular immune
responses in rats. The ESA can depress the proliferative response of spleen mononuclear cells and inhibit nitric oxide
(NO) production by peritoneal cells. In the present study we identified ESA proteins of ca 24 kDa, which shared significant
sequence homology to glutathione-S-transferase (GST) obtained from homogenates of F. hepatica adults, other helminths
and different mammals. When the dimeric form of these proteins ca 48 kDa was cultured with rat spleen cells, a significant
decrease of proliferative response to Con A was detected, starting from 20 μg/ml of ESA protein (P<0·03). We also
observed a significant inhibition of nitrite production by incubation with the dimeric form in normal peritoneal
macrophages (P<0·04). These results indicated that the GST secreted by the parasite could be involved in evasion of
the parasite from the host immune response.