In an attempt to gain insight into the biosynthesis and processing of human Thy-1, rabbit antisera (R alpha TP) were raised against a synthetic peptide (TP) of 13 amino acid residues of identical amino acid sequence to that of residues 110-122 of the putative Thy-1 precursor deduced from the cDNA sequencing. Immunoblotting assay showed that the IgG fraction of R alpha TP (R alpha TP-IgG) recognized the synthetic peptide without demonstrable cross-reactivity with isolated human brain Thy-1 and detergent-solubilized membrane proteins of human brain cells. Immunohistochemical studies showed that both R alpha TP-IgG and HB-2S-1 (a monoclonal antibody which reacts with both membrane-bound Thy-1 on viable cells and detergent-solubilized Thy-1) stained cells of two human T lymphoma cell lines (HUT-78 and HUT-102) but they did not stain cells of a human B lymphoma cell line (Raji). In contrast, in cell surface indirect immunofluorescence assay, HB-2S-1 reacted with HUT-78 and HUT-102 cells while R alpha TP-IgG did not. Taken together, these data indicate the existence of a precursor form of human Thy-1 which is processed prior to anchoring to the cell surface.