Chemicals present in bleached kraft pulp mill effluents (BKMES) can modulate the immune response of fish. This work was conducted to determine if mummichogs living downstream from a bleached kraft pulp mill (BKM) had an impaired immune response. Head kidney phagocytes from males and females were tested for their phagocytic and metabolic activities. Mummichogs were captured 4 km (S2) and 39 km (S1) downstream from a BKM in the Miramichi estuary (New Brunswick, Canada) and at two sites located upstream (S3) and 20 km downstream (S4) in a reference estuary, the Bouctouche. Both male and female mummichogs captured at S2 in the Miramichi estuary had a lower phagocytic activity, whereas only females from the same site had a decreased oxidative burst response, indicating a difference between sexes in the susceptibility to pollutants present in the Miramichi. Therefore, the results indicate that mummichogs exposed to BKMES have an impaired immune response and that females are more sensitive than males to the chemicals present in BKMES.