The cellular response, within the kidney of experimentally infected carp to the presence of eggs of the blood fluke Sanguinicola inermis has been investigated using transmission electron microscopy. Initially, 0-1-week-old eggs were surrounded by host eosinophils which degranulated and broke down adjacent to the egg shell. Infiltration by neutrophils and monocytes occurred around 1-2-wcek-old eggs. Eggs at 2 -3 weeks of age began to become encapsulated, although the first signs of structural damage appeared when the eggs were 5-6 weeks of age.