The influence of serum immunoglobulins on the incidence of calf pneumonia, and the relationship between sub-clinical pneumonia, and calf serum Ig levels, is discussed. Monospecific antisera were used to measure levels of IgG1, IgG2, IgM and IgA in the sera of bull calves aged around 2 1/2 weeks. These calves were selected retrospectively according to their disease record up to six months of age. A clear association was found between low levels of IgG1, IgG2 and IgA in the "2 1/2-week" sample and subsequent susceptibility to pneumonia at around 2 1/2 months of age. Calves showing signs of pneumonia had low levels of IgG1 (45-5 per cent had less than 8 mg per ml compared with "now-pneumonic" calves which had relatively high levels (only 9.5 per cent had less than 8 mg per ml). In contrast, older calves bled at monthly intervals and found to have pneumonic lesions at slaughter had significantly evevated levels of both IgGa and IgG2.