The activation of natural killer (NK) cells and induction of cytotoxicity are complex processes whose molecular mechanisms have not been clearly elucidated. Stimulation of the NKL human NK cell line with interleukin-2 (IL-2) or protein-bound polysaccharide K (PSK) leads to sustained growth and cytolytic activity in comparison to unstimulated NKL cells. Our previous results shown that IL-2 and PSK regulate different nuclear transcription factors in NKL cells, and that the signal transduction pathway used by these inducers is different. To determine the molecular basis for the different action of IL-2 and PSK, we investigated the upstream effects generated in human NKL cells by IL-2 and PSK on protein kinase C (PKC) isoenzymes and mitogen-activated protein kinases (MAPK). Here we report the profile of unstimulated NKL cells as: PKCbeta>PKCalpha>PKCdelta =PKCepsilon. The PKCeta form was not expressed. The effects of PSK and IL-2 on these isoenzymes were different. IL-2 increased the expression of PKCalpha, PKCdelta and PKCepsilon, whereas PSK decreased the expression of PKCalpha, and also increased PKCdelta and PKCepsilon to higher levels than did IL-2. In MAPK expression we found that unstimulated NKL cells have the following profile: ERK2>ERK6>p38gamma>p38beta>ERK1. ERK3, ERK3 rel, ERK5/ERK4 and p38delta were not expressed. IL-2 decreased the expression of ERK2, whereas PSK did not, and both agents increased the expression of ERK3. These results shown that PSK and IL-2 produce different variations in PKC isoenzymes and MAPK in NKL cells.