Genetic and phenotypic parameters of immune response traits were investigated for Holstein bulls. Aspects of general immune response were evaluated in 60 post-pubertal bulls. Assessment of neutrophil function was emphasized and was based on laboratory measures of migration, phagocytosis, oxidative metabolism, and cytotoxicity. Lymphocyte blastogenesis due to mitogens and leukograms were included in the battery of laboratory assays. Data were collected from bulls before, during, and after glucocorticoid-induced immunosuppression in a biological model that mimicked periparturient immunosuppression. Derivative-free maximum likelihood was used to estimate genetic and phenotypic (co)variances for immune response traits. Traits were defined as immune function measured by a specific laboratory assay on several days within a particular time period. Single- and multiple-trait animal models were used in the data analyses. Prior to immunosuppression, heritabilities for 14 immune response traits ranged from .27 to .56. Heritabilities in weeks 2 and 3 tended to be lower than week 1, although heritabilities for spontaneous lymphocyte blastogenesis, neutrophil iodination, directed migration, and cytotoxicity assays varied little across weeks. Genetic and phenotypic correlations between traits described by data from the same assay across different weeks generally ranged from .30 to .75 and .25 to .60, respectively. Genetic correlations among measures of neutrophil oxidative metabolism and between phagocytic capacity and neutrophil cytotoxicity were high and positive. Immune responses during stressed and non-stressed periods can be generally characterized as separate but overlapping traits. Estimated parameters for immune response could potentially be used for construction of selection indices for greater disease resistance.