Acid treatment of bovine lymphocytes by a buffered solution of 0.263 m citric acid and 0.123 M Na2HPO4 at pH = 3.0, originally described for human and murine lymphocytes, selectively eliminated the antigenicity of MHC (BoLA) class I determinants also from bovine lymphocytes. The viability of acid-treated cell suspension was higher than 90%. The reactivity of acid-treated lymphocytes with BoLA class I typing alloantisera was lost in microcytotoxicity test, while their reactions with cross-reactive anti-HLA class II, anti-BoCD2 and BoCD5 monoclonal antibodies, and with antisera detecting two non-MHC lymphocyte alloantigenic specificities (BoLY w1 and R') remained unchanged in the microcytotoxicity and/or indirect immunofluorescence tests. The results thus show that this approach of modulating cell surface expression of MHC class I determinants may be used in cattle.