The "multidrug resistance" (MDR) system involves the action of transmembrane P-glycoproteins (Pgp) which may be responsible for failure of chemotherapy in both invertebrates and vertebrates. We previously obtained partial reversion of anthelmintic resistance in nematodes subjected to both anthelmintics and inhibitors of this system. The results presented here are able to describe more accurately the presence of Pgp in nematodes because of the use of C219 and UIC2 monoclonal antibodies, which are used for the detection of human and mouse mdr1 gene products. These antibodies demonstrated the presence of Pgp in eggshells. Their role in these structures, which are considered to be passive barriers, remains to be determined. Flow cytometry analyses of the UIC2 staining allowed determination of the resistance of individuals, which varied within the parasite population. UIC2 demonstrated both the presence and activity of Pgp in nematodes as has previously been shown in tumour cells. Resistance seems to be due to an increase in both the number of Pgp sites and parasites with high levels of Pgp.