Development of immunity to Ostertagia ostertagi and Cooperia oncophora and interactions between both species in primed calves were investigated after homologous, heterologous and concurrent challenge infections. Worm counts, faecal egg output, IgG1 and IgG2 antibodies and the presence of globule leucocytes were used to evaluate the possible interactions. Results show that immunity build-up against O. ostertagi is slow in comparison with C. oncophora. The presence of early fourth-stage larvae and globule leukocytes in the O. ostertagi primed groups was significantly different to that of a previously uninfected control group. Ostertagia ostertagi and C. oncophora IgG1 antibodies were high in the previously exposed groups compared with uninfected controls and C. oncophora antibodies cross-reacted strongly with O. ostertagi antigens. There was no conclusive evidence for an interaction between C. oncophora and O. ostertagi. Globule leukocytes, IgG1 antibodies and early fourth-stage larvae seem to be related to development of immunity to O. ostertagi.