Right mammary quarters of dairy cows were infused with concanavalin A (conA) or phytohemagglutinin (PHA) near drying off. Significant changes in milk yield and composition were observed by 24 hours after infusion. In general, conA or PHA treatment resulted in significantly greater compositional changes earlier in the dry period compared with control quarters. As involution progressed, lactoferrin, serum albumin, immunoglobulin G, pH, and numbers of somatic cells increased, whereas citrate and the molar ratio of citrate:lactoferrin decreased. Mammary secretion accumulation at 7 days of involution was significantly lower in treated compared with control quarters. The data indicated that intramammary infusion of conA or PHA near drying off accelerates bovine mammary involution, resulting in elevated levels of natural protective factors.