The results of 4 different assays for haemocytic immune responsiveness in the cockroach, Periplaneta americana, have shown that the presence of larvae of Moniliformis moniliformis within the haemocoele depresses the insect's immune reactivity. Thus, the proportion of haemocytes that phagocytose fluorescent latex beads in vivo, the proportion of haemocytes that can be stimulated in vitro by the β1,3-glucan, laminarin, to produce phenoloxidase, and the number of haemocytic aggregates produced in vivo in response to zymosan stimulation, are significantly depressed compared with control, unparasitized cockroaches. Also, when cockroaches are injected intra-haemocoelically with hatched oncospheres of the tapeworm, Hymenolepis diminuta, a higher prevalence and intensity of tapeworm larvae are found in insects already parasitized by M. moniliformis. All of these assays show that depression rather than total suppression of the immune response occurs; in the wild, this may be sufficient to ensure that the cockroach does not succumb to unrelated infections during the long developmental period of the parasite.