Globule leucocytes (GL) occur in the respiratory tract of cattle from foetuses to adult animals. Large numbers of this cell have been found in the lungs of cows both in normal lung and in the lungs of cows from various outbreaks of respiratory diseases, but the significance was not known. In the rat and in cattle, the occurrence of GL in the gastrointestinal and urinary tracts has been associated with parasitic infections. The relationship between the respiratory tract GL and Dictyocaulus viviparus infections in cattle was studied in three groups of parasite free calves experimentally infected with D. viviparus. Group 1 animals received single challenge infections. Group 2 had three subgroups (2a, 2b, 2c) which were either vaccinated and challenged (2a), given a single challenge (2b) or used as vaccinated controls (2c). Group 3 with four subgroups (3a, 3b, 3c, 3d) were either used as environmental controls (3a), vaccinated and repeatedly challenged (3b), given normal larvae of D. viviparus and repeatedly challenged (3c) or as a trickled group 3d. Although GL were found in infected calves, it was not possible to demonstrate a clear cut correlation between their presence or density and the method of exposure.