The development of immunity to Ostertagia ostertagi and Cooperia oncophora and interactions between these species were investigated in experimentally infected calves. Parasitological, serological and histological parameters were used for assessing immune responses. No conclusive evidence of an effect of C. oncophora on the course of an O. ostertagi infection in calves could be shown. Following a challenge with C. oncophora and O. ostertagi of C. oncophora primed calves, no significant reductions in establishment rate, faecal egg counts, worm length or the percentage of early fourth stage larvae could be demonstrated. Results also confirmed earlier work showing the very different degrees of immunity conferred following immunisation with either C. oncophora or O. ostertagi. While a protective immunity was generated in the case of C. oncophora, continuous infection of calves with 420000 L3 of O. ostertagi during almost 5 months induced immune reactions which affected growth and fecundity of the worms but not the establishment rate.