Lactoferrin and Ig are the major glycosylated proteins in whey preparations from colostrum, milk, nonlactating bovine mammary secretions, and milk collected after intramammary endotoxin challenge. Lactoferrin was isolated from these sources and from bovine mammary tissue (nonlactating) and bovine milk neutrophils. Molecular weight forms of isolated lactoferrins were separated by SDS-PAGE. Apparent molecular weight forms of lactoferrin (approximately 83 and 87 kDa) did not differ among lactoferrins isolated from mammary secretions or from mammary tissue, but lactoferrin isolated from milk neutrophils migrated as different molecular weight bands in the gels (approximately 87 and 91 kDa). Human milk lactoferrin also separated as two distinct bands in the gels. All forms of lactoferrin were glycosylated. Differences were distinct in the glycosylation pattern of lactoferrins from human milk, bovine nonlactating mammary secretion, and bovine milk neutrophils. Enzymatic deglycosylation of lactoferrins from those sources resulted in migration of each as a single band (approximately 77 kDa). Apparent molecular weight forms of lactoferrin observed by separation by SDS-PAGE are not the result of genetic variance or differential glycosylation at different stages of mammary gland function. Nevertheless, the forms of lactoferrin result from the presence of glycans on the protein.