Abstract Sixteen lactating cows were assigned to two groups in a 28-day switchback design to compare the nutritive value of water treated and .9% formaldehyde treated soybean meal. During the last 7 days of each period four cows from each group were used in total collection digestibility and nitrogen balance trials. Intake of dry matter and nitrogen, and milk production and composition for control and treated groups were: dry matter, 13.9 and 13.9 kg/day; nitrogen intake, 288 and 277 g/day; 4% fat-corrected milk, 17.2 and 17.1 kg/day; milk fat, 3.84 and 3.90%; and milk protein, 3.51 and 3.43%. Apparent digestibility coefficients, total digestible nutrients, and productive nitrogen (milk nitrogen + retained nitrogen) for control and treated groups were: dry matter, 67.3 and 65.1%; organic matter, 68.3 and 66.2%; crude protein, 64.2 and 57.2%; acid detergent fiber, 41.9 and 38.9%; ether extract, 62.0 and 62.0%; nitrogen-free extract, 77.0 and 75.7%; total digestible nutrients, 67.5 and 65.7%; and productive nitrogen, 108 and 96 g/day. Feeding a diet containing formaldehyde treated soybean meal to lactating cows did not significantly increase free amino acid concentrations in arterial or venous plasma or amino acid uptake by the lactating mammary gland as compared to feeding water treated soybean meal in the same basal diet. From amino acid uptake by the mammary gland and output in milk protein, histidine, phenylalanine, methionine, lysine, and threonine were the five essential amino acids used most completely for milk protein synthesis. Therefore, these amino acids may be the most critical in limiting production of milk protein. Feeding formaldehyde treated soybean meal to lactating cows had no beneficial effect on milk production or milk protein synthesis; however, the protein may have been overtreated with formaldehyde.