The composition of sow colostrum and milk was quantitated in 25 sows at 14 time points throughout lactation. All animals belonged to the same experimental herd of German Landrace, farrowed within 4 d, and were of various lactation numbers and various litter sizes. In the first 6 h of lactation colostrum total solids (TS) and protein contents were higher, while fat and lactose contents were lower than in mature milk. Decreased total protein and whey protein contents and concomitantly increased fat and lactose content, with nearly unchanged TS levels, indicate transition from colostrum to mature milk. The high protein content of colostrum was largely due to immunoglobulin (Ig). During the first 6 h, IgG accounts for nearly all the protein in colostrum but plays a decreasing role in sow milk as lactation proceeds. After 2 wk, IgA levels begin to increase and at the end of lactation, IgA constitutes 40% of the total whey protein. No influences of lactation number and litter size on milk composition could be ascertained in this study.