The concentrations of IgG, IgM and IgA in sera collected from 3855 sows (3208 pregnant and 647 lactating) at a single time point were determined. This experimental design allowed changes in serum immunoglobulin over the reproductive cycle to be studied without bias from seasonal influence. The concentrations of the three immunoglobulins changed independently during the reproductive cycle. Serum levels of IgM and IgG began a progressive postpartal decline during the 14th-17th week of gestation. At the onset of lactation serum IgG levels progressively increased while IgM levels continued to decline, the latter reaching their lowest level during the third week of lactation. In contrast to IgM and IgG, serum IgA levels increased 35% during weeks 14-17 of gestation and continued to increase throughout lactation, reaching their highest serum levels in the third week of lactation; the serum IgA concentration at this time was twice that observed during the first 13 weeks of gestation. Results of these studies allowed the reproductive cycle to be classified into four phases on the basis of serum immunoglobulin concentrations: (1) weeks 1-4 of gestation; (2) weeks 5-13 of gestation; (3) weeks 14-17 of gestation and (4) lactation.