CS1 (CRACC, novel Ly9) is a novel member of the CD2 family expressed on natural killer (NK), T and stimulated B cells. Although the cytoplasmic domain of CS1 contains immunoreceptor tyrosine‐based switch motifs (ITSM), which enables to recruite signaling lymphocyte activation molecule (SLAM)‐associated protein (SAP/SH2D1A), it activates NK cells in the absence of a functional SAP. CS1 is a self ligand and homophilic interaction of CS1 regulates NK cell cytolytic activity. Here we have identified a novel splice variant of CS1 (CS1‐S), which lacks ITSM. Human NK cells express mRNA for both wild‐type CS1 (CS1‐L) and CS1‐S and their expression level remained steady upon various stimulations. To determine the function of each isoform, cDNA for CS1‐L and CS1‐S were transfected into the rat NK cell line RNK‐16 and functionally tested using redirected cytotoxicity assays and calcium flux experiments. CS1‐L was able to mediate redirected cytotoxicity of P815 target cells in the presence of monoclonal antibody against CS1 and a rise in intracellular calcium within RNK‐16 cells, suggesting that CS1‐L is an activating receptor, whereas CS1‐S showed no effects. Interestingly, SAP associated with unstimulated CS1‐L and dissociated upon pervanadate stimulation. These results indicate that CS1‐L and CS1‐S may differentially regulate human NK cell functions.