The phospholipase B activity was assayed in the small intestines, spleen and liver/bile duct of nonsensitized and sensitized mice infected with Fasciola hepatica. The primary infection resulted in a significant increase in phospholipase B activity in the small intestine, spleen and liver/bile duct over that of uninfected control animals. The response to the challenge infection was characterized by an earlier increase in enzyme activity with values significantly above those found for the primary infection in the same tissues. These data demonstrate that one response of mice to infection with F. hepatica is characterized by an increased phospholipase B activity. Thus, the enzyme response is not unique to cestode and nematode infections, but also is part of or associated with the inflammatory mechanisms against trematode infections. In addition, phospholipase B was demonstrated in leukocytes using electron microscopic histochemical techniques. Leukocytes were harvested from peritoneal exudates of mice. Cells were fixed in 4% calcium-forol fixative for 30 minutes at roan temperature for electron microscopy, after which they were incubated at 370C in medium at pH 6.6 containing 2 aM lysolecithin and Cal2 . The fatty acids released during the hydrolytic reaction were trapped as a calcium precipitate and were converted to a …