The sequential development of Type I and Type II ostertagiasis over a 2-year period in the same naturally infected cattle is described for the first time. Particular reference is made to biochemical and serological changes. Positive relationships were demonstrated between the clinical signs of both Type I and Type II disease, and marked increases in the levels of plasma pepsinogen, plasma gastrin and antibody titres to adult Ostertagia antigen. At necropsy, there were significant relationships between the combined total of adult and developing 5th stage larvae of Ostertagia spp. and the levels of both plasma pepsinogen and gastrin. By the end of the second grazing season the cattle had acquired an immunity to infection with Ostertagia spp. and had very low burdens of this parasite at necropsy. However some of these cattle maintained elevated plasma pepsinogen levels when under natural challenge by Ostertagia spp. larvae and the aetiology of these changes and the problems of diagnosis using this parameter are discussed. Similar trends of infection were observed for Cooperia oncophora, although resistance to the parasite developed more rapidly.