Guinea pigs were fed for 14 days with semisynthetic diets containing protein (milk casein: 1.5, 30, 45% and soya protein: 4, 18 and 36% of total energy) or with different amounts of L-methionine (Met) and glycine (Gly). Simultaneously with diets feeding the animals were sensitized epicutaneously by formaldehyde (F). The sensitization to F was evaluated by leukocyte specific lysis reaction (RSLL). Hepatic formaldehyde dehydrogenase (FDH) activity and reduced glutathione (GSH) content were measured. The sensitization to F decreased when the animals were fed with growing amount of plant or animal protein, or with high amount of Met and normal level of Gly. FDH activity increased in sensitized animals compared to non-sensitized irrespective of the diet fed. Hepatic GSH pool significantly grew with an increase in both proteins level and in sensitized animals also with growing Met and Gly intake. GSH pool did not change in sensitized animals fed with different amounts of amino acids. It is suggested that high dietary protein and amino acids protection against F-sensitization was mediated by elevated tissue GSH though optimal consumptions of Met and Gly were different. Hepatic FGH activity together with RSSL may be considered as a sensitive biochemical criterion of experimental F sensitization.