The effect of methanal (HCHO) treatment of casein on its in vitro and in vivo digestion was investigated. As the quantity of HCHO bound to casein increased, the in vitro rumen degradability decreased but no further reduction occurred above 10.6 g HCHO kg-' treated casein DM. The proportion of the original HCHO (25.4 g HCHO kg-I treated casein DM) remaining bound to casein at pH values 3.0 to 6.0 was approximately 0.5; the proportion remaining bound at pH 2.0 was significantly less (0.39). N solubility and in vitro digestion of casein by pepsin/HCI andlor intestinal proteolytic enzymes was significantly reduced by treatment with HCHO (10.6 g HCHO kg-1 treated casein DM). When fed to rats the dry matter (DM) and true N digestibilities decreased significantly as the ratio HCH0:casein increased; the digestibility of the N-free component of the diet remained unchanged. The quantity of available lysine in the diets and their true biological value (BV) decreased as the level of bound HCHO increased; there was significant linear relationship between BV and available lysine. The apparent disappearances of amino acids from the small intestine of sheep, when untreated or HCHO treated caseins (12.3 g HCHO kg-' DM) were infused into the duodenum, were not affected by the pH of the infusate (pH 2.5 or 6.8) but were significantly reduced by HCHO treatment. It is concluded that exposure of HCHOtreated casein to low pH values does not completely release bound HCHO from casein and that the intestinal digestion of casein to which HCHO remains bound is reduced when compared to untreated casein. All amino acids determined showed similarly reduced apparent intestinal absorptions due to HCHO treatment. It is suggested that the lower BV of HCHO-treated caseins was due to reduced metabolic availability of lysine.