We previously described a lymphokine termed HILDA (for human interleukin DA) produced by T-lymphocyte alloreactive clones after antigenic stimulation. This factor sustains the growth of a murine IL3- sensitive cell line (DA2). In addition, HILDA is a potent activator of eosinophils and displays a burst-promoting activity on human bone marrow. In the present study, HILDA was purified to homogeneity from T- cell clone supernatant using successively sequential concentration, concanavalin A (ConA) affinity chromatography with differential elution (alpha-D glucopyranoside and alpha-D mannopyranoside), high-performance liquid chromatography (HPLC) gel filtration and reverse-phase HPLC. The pure material appeared as a 38-kd glycoprotein on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under reducing or nonreducing conditions. Biologic activity could be recovered from SDS- PAGE gel slices corresponding to the 38-kd band. We conclude from the specificity of the DA-2 cell line and biochemical characteristics described that this lymphokine is different from other known factors produced by human T lymphocytes.