An absorbed antiserum was made by incubating a cytotoxic T lymphocyte immune antiserum with the T cell leukemia HPB-ALL, thus removing reactivity to known lymphocyte function-associated antigens. This antiserum was used to screen a cDNA expression library and isolate a novel human lymphocyte cDNA clone designated WP34, WP34 transcript is expressed in functional T cells and a variety of hematopoietic cell lines and tissues, including fetal liver and thymus, but not in HPB-ALL or any non-hematopoietic cell lines or tissues tested. The WP34 protein is an acidic, phosphorylated molecule with a pI of 4.5 and molecular mass of 50 kDa. WP34 protein expression is absent in resting peripheral blood lymphocytes but can be induced with antigen stimulation, while the transcript is constitutively expressed. Sequence analysis indicates that WP34 is the human homologue of the recently described murine molecule LSP1.