Cultured natural killer (NK) cells derived from CD3- CD56+ high-density small lymphocytes (HDLs) exhibit similar morphology and high levels of non-major histocompatibility complex-restricted (NK) cytotoxicity equivalent to those of cultured NK cells from CD3- CD56+ low-density large granular lymphocytes (LGLs). To examine the similarities and differences between NK cells from HDLs and NK cells from LGLs, we investigated the expression of three distinct members of the granule serine protease (granzyme) family within cultured CD3- CD56+ LGLs and HDLs. CD3- subpopulations of nonadherent peripheral blood mononuclear cells, LGLs (density < 1.063 g/ml), and HDLs (density > 1.063 g/ml) were stimulated to proliferate in culture. The cultured cells from each population were entirely CD3- CD56+ and were indistinguishable in terms of their increased granularity and size once activated. All cultured CD3- CD56+ LGLs and HDLs displayed cytolytic activity against K562 and immunoglobulin-coated P815. Western analysis detected perforin in both cultured LGL and HDL populations. Cultured HDLs and LGLs both expressed BLT-esterase activity and human granzyme A mRNA. Granzyme B mRNA and protein and Asp-ase activity were detected in unstimulated and cultured LGLs and cultured HDLs. By contrast, unstimulated HDLs did not express significant levels of granzyme B. High levels of Hu-Met-1 granzyme mRNA and Met-ase activity were detected only in cultured LGLs. Thus, despite the development of large granular morphology during proliferation, interleukin-2 cultured CD3- CD56+ HDLs display a different pattern of granzyme expression from CD3- CD56+ LGLs. These data also further suggest an unusually restricted expression of the Hu-Met-1 granzyme in LGLs.