The nutritional value of soybean meal that had been treated with formaldehyde (.3 g/100 g) to inhibit microbial degradation of soybean meal protein in the rumen was investigated. Four experimental diets were fed ad libitum during wk 4 to 43 of lactation to Holstein cows randomly assigned to diets in a 2 × 2 factorial arrangement of dietary crude protein (12 and 14%) and formaldehyde treatment (untreated and treated soybean meal). Concentrate, corn silage, and alfalfa-grass hay provided 53.0, 35.4, and 11.6% of the daily intake of dry matter. Analysis of covariance revealed that digestibility of dietary crude protein by cows fed formaldehyde treated soybean meal was lower than by cows fed untreated soybean meal (62.4 versus 65.4%). Similar quantities of milk, 4% fat-corrected milk, milk fat, and milk solids-not-fat (overall means of 7998, 7402, 281, and 660 kg/301 days of lactation) were produced by cows fed different diets. This was true whether the data were summarized during peak production (day 22 to 63), during days 22 to 119 when crude protein intake did not meet requirements, or during the complete experiment (days 22 to 301 of lactation). Milk protein (total nitrogen × 6.38) produced by cows fed soybean meal treated with formaldehyde was less than by cows fed untreated soybean meal during days 22 to 63 and during days 22 to 119 (47 versus 44 kg/cow and 103 versus 97 kg/cow). Changes in body weight of cows during lactation were similar among treatments. Treating soybean meal with .3 g formaldehyde/100 g may decrease availability of soybean meal protein for use by lactating dairy cows.