Fresh sublingual gland tissue from Macaco irus monkey was minced and incubated at 37 °C for 72 h in a synthetic medium supplemented with radioactively labelled L-leucine, D-glucose, or both. The secreted proteins were separated by isoelectric focussing from the culture medium (LKB 8101 column, ampholines pH 3–10). The radioactive incorporation was determined by scintillation counting of the dialyzed fractions. Five labelled sublingual protein fractions were found, with isoelectric points of about 3.0, 4.7, 6.0, 7.5, and 10.0. The most consistent fractions were No. 1, 2, and 5. The acidic fraction (pI 3) was identified as a virus-haemagglutination-inhibiting glycoprotein (HI) containing sialic acid, previously isolated from human saliva. A molecular weight of about 600,000 was found by ultra-centrifugation after further purification of this fraction. The largest protein fraction (pI 4.7) homogeneous as judged by disc electrophoresis, was also of glycoprotein character, with amino acid and amino sugar patterns similar to the HI glycoprotein. Carbohydrate analysis, however, indicated only fucose, not sialic acid, as the terminal carbohydrate. The alkaline fraction (pI 10.0) showed a different amino acid content and contained only small amounts of amino sugars. The possibility of finding the pI 4.7 glycoprotein fraction in other salivary glands is discussed.