We describe a new rat immunoglobulin M monoclonal antibody (CH-F42) that recognizes a subset (1.5% to 8%) of normal peripheral blood T lymphocytes. The phenotype of these cells was determined, using dual-color immunofluorescence, to be CD2+, CD3+, CD4+, CD5+, CD7-, CD8-. They do not express T-cell activation markers, and are positive for UCHL1 (CD45RO), but negative for 2H4 (CD45RA). The antige...