Abstract: The purpose of this study was to investigate the effects of cysteamine compound (Lactonin) on milk production and immune function of dairy cows during week (W) 20 through 42 of lactation. 96 Holstein lactating cows at mid- and late-lactation stage were assigned to 4 groups (n=24) on the basis of their daily milk yield (M) prior to the experiment. In each group the cows were divided randomly into Lactonin treatment (LT, n=49) and Control (n=47). In LT， Lactonin was administered to cow progressively from 2 000 to 6 000 U/d·head through the experimental period.In G1 cows received LT (n=12) produced 18% more milk (P<0.05) than did Control during the entire 23 weeks of treatment period. But in G2, G3 and G4 the milk yield of cows treated LT did not differ from that of Control, suggesting that the effect of Lactonin on milk production was influenced by the basic milk yield of cows in the period prior to Lactonin treatment. In LT groups（n=49） the milk fat (3.64%±0.05% vs 3.78%±0.05%, P<0.05) and protein percentage (2.94%±0.04% vs 3.02%±0.03%, 0.05<P<0.15) were greater than those in Control(n=47). In LT of G4 (n=13) milk fat and protein increased by 11% (P<0.05) than that of in Control, respectively. LT treatment tended to increase FCM（0.05<P<0.15）in G1 and G4. The immune activities, determined by lymphocyte transformation rate, IL-2 and IL-6, were enhanced (P<0.05) in LT treatment group (n=32) in the W 24 of lactation. Results from this study indicate that LT improved the milk yield and milk composition in the mid and late-lactation stages. These effects of Lactonin on milk production are related to its promoting effects on dairy cows in digestion, absorption and partition of the nutrients to the tissues, and its stimulating effects on immune function.