The objective of this experiment was to investigate the effect of an enzyme extract (ENZ) from Aspergillus oryzae and Aspergillus niger on lactational performance, blood metabolites, and nutrient digestibility in dairy cows. Forty-eight Holstein cows (days in milk, 139 ± 51.6 d; body weight, 621 ± 72.8 kg) were used in a 10-wk randomized complete block design experiment. Cows were blocked based on days in milk, milk yield, and parity following a 2-wk covariate period. Cows within a block were randomly assigned to one of the following treatments (24 cows/treatment): (1) control and (2) ENZ supplemented to the basal diet at 4.2 g/kg feed dry matter. Cows were housed in a free-stall barn equipped with Calan Feeding System for monitoring individual feed intake and fed ad libitum once daily. The basal diet consisted of 60% forage (corns silage and alfalfa haylage) and 40% concentrate feeds. ENZ was mixed with the total mixed ration daily, before feeding. Milk production and dry matter intake (DMI) were monitored daily. Milk samples for composition analysis were collected during wks 4, 6, and 8. Blood samples for analysis of glucose, nonesterified fatty acids, and β-hydroxy butyrate were collected during wk 6. Fecal samples for determining total-tract apparent digestibility of nutrients were also collected during wk 6. Data were analyzed using the MIXED procedure of SAS with treatment, week [for repeated measures variables using ar(1) covariance matrix], and interactions in the model. Block and treatment × block were random effects. Compared with the control, ENZ increased (P = 0.04) DMI (23.6 and 24.7 kg/d, respectively; SEM = 0.35) and milk yield (32.4 and 33.6 kg/d; SEM = 0.56) but had no effect on energy-corrected milk yield. ENZ also increased (P < 0.01) milk protein and lactose concentrations and yields and decreased (P < 0.01) somatic cell count. Treatment had no effect on blood metabolites and did not affect apparent total-tract digestibility of nutrients. In this experiment, dietary supplementation of ENZ increased DMI and milk and milk protein yields in dairy cows.