Our recent work on the gene structures for human salivary (S-type) cystatins [ Saitoh, E. et al. (1987) Gene 61, 329-338] has suggested that the structures of cystatins which we determined previously at the protein level lack N-terminal peptide portions of the fullsized intact forms. In the present study, attempts were made to isolate full-sized S-type cystatins by introducing methanol fractionation into the purification steps to suppress the enzymatic activity present in saliva. Full-sized cystatin SN and two phosphorylated forms of full-sized cystatin S were thus isolated. Analysis of one fraction indicated that this was a mixture of full-sized cystatin SA and non-phosphorylated cystatin S. The phosphorylation sites of cystatin S were determined to be Ser-Ser-Ser¹(P)-Lys-Glu-Glu- for monophos-phorylated cystatin S and Ser¹ (P)-Ser-Ser³(P)-Lys-Glu-Glu- for diphosphorylated cystatin S. Immunoblotting analysis with anti-cystatin S antiserum revealed that tears and seminal plasma also contained S-type cystatins, but diphosphorylated cystatin S was detected neither in tears nor in seminal plasma and no cystatin SN was found in seminal plasma. These data indicate that S-type cystatins are secreted into the oral cavity without significant degradation in salivary glands or ducts and that they are expressed tissue specifically.