Eukaryotic parasites residing in earthworms such as Lumbricus terrestris, have been studied for many years. However, little research has been published on the host-parasite relationship, particularly the immune response of L. terrestris and the effect of the parasite on the host. The majority of individuals belonging to the species Lumbricus terrestris are parasitized by a sporozoan of the genus Monocystis, the spores of which can be found in huge numbers in the worms' seminal vesicles. This preliminary study of the Lumbricus terrestris-Monocystis relationship attempts to determine whether a cellular reaction by the earthworm's coelomocytes (immune cells) occurs both in vivo and in vitro. Light and electron microscopic observations indicate that there is no apparent interaction between coelomocytes and parasites in the worms' seminal vesicles. However, in vitro incubations of the Monocystis spores together with coelomocytes produced an encapsulation response: a rapid attachment to and aggregation of coelomocytes around parasites, followed by a complete degranulation of the coelomocytes. The degree of degranulation was monitored by measuring acid phosphatase levels, an enzyme released during degranulation. These were subsequently compared to incubations with equal numbers of other eukaryotic parasites with thick cyst walls (Giardia and Cryptosporidium species), but which do not normally infect the worm. Acid phosphatase levels were significantly higher in Monocystis incubations compared to those of G. lamblia and Cryptosporidium, indicating a greater cellular response to the endogenous parasite. Interestingly, Giardia lamblia cysts actually inhibited degranulation compared to that of control (spontaneous degranulation) coelomocytes, a phenomenon possibly due to the nature of the cyst wall. These experiments show that whereas an immune response against the predominant spore stage of Monocystis does not occur within the seminal vesicles of the earthworm, the hosts' coelomocytes are capable of responding to these spores in vitro, and do so to a greater degree than to non-native parasites.