The effects of individual variation, season and parasite activity on the levels of the four major immunoglobulins, IgM, IgA, IgG₁ and IgG₂, were analyzed in four adult Rocky Mountain bighorn ewes. Individual variation was significant for all immunoglobulin classes. Seasonal effects were detected in the levels of IgG₁. The fecal count of Protostrongylus larvae was inversely related to IgG₂ concentrations and the fecal Nematodirus count was inversely related to IgA. Muellerius and ccccidia did not demonstrate significant relationships with circulating levels of any of the immunoglobulin classes.