Levamisole administered by oral, subcutaneous or transdermal routes following vaccination against Pasteurella multocida enhanced the humoral response in cross-bred calves. The group that received levamisole subcutaneously elicited highly significant (P less than 0.01) antibody titres during the primary humoral response in comparison with groups that received levamisole orally or transdermally. During the secondary response in the same experiment, levamisole administered subcutaneously and transdermally showed highly significant (P less than 0.01) haemagglutination titres relative to oral administration of the drug. In a second experiment, the group that received levamisole subcutaneously showed highly significant (P less than 0.01) antibody titres during the anamnestic humoral response over other treated groups.