Using monospecific sheep antisera, the mean plasma immunoglobulin concentrations in 82 Ayrshire milking cows were found, by the single radial diffusion method, to be: IgG2 10-91 plus or minus 4-30, IgG1 9-76 plus or minus 2-59 and IgM 2-71 plus or minus 1-22 mg/ml (plus or minus SD). IgA levels in the same group of 82 cows were determined by a simple, sensitive coated tube radioimmunoassay and had a mean value of 0-32 plus or minus 0-16 mg/ml. The frequency distributions of IgG1, IgM and IgA were approximately log normal, whereas that of IgG2 was bimodal with two populations with means of approximately 7 and 14 mg/ml. An 18-fold difference was found between high and low IgG2 levels whereas only three- to four-fold differences were recorded for IgG1 levels in the same population. Eight-fold differences were found for IgM and IgA levels. The effect of age (2-9 years) on sampling could be disregarded for IgG1, IgG, IgM and IgA levels, but cows above 6 years of age had significantly higher IgG2 levels.