Thy-1 antigen expression in the rat thymic myoid cell line R615B2 (Thy-1(+] and R613Ad (Thy-1(-] was studied with a semi-quantitative immunohistochemical assay without disrupting the cell cultures. With this assay, the quantity of Thy-1 antigen on R615B2 cells was detected separately in the cytoplasm and on the cell surface by the use of appropriate fixatives such as chilled ethanol and methanol + 0.3% H2O2. Extracellular Thy-1 antigen was also found in the culture supernatant of R615B2 cells. More than half of extracellular Thy-1 antigen remained in the supernatant even after 100,000 X g centrifugation. No form of Thy-1 antigen was detected at significant levels in R613Ad cells.