The present invention relates to an immunostimulatory peptide derived from an Hsp70 protein and peptides comprising said immunostimulatory peptide. Furthermore the present invention pertains to polynucleotides encoding said peptide, vectors comprising said polynucleotides, fusion (poly)peptides comprising said peptide and compositions comprising said peptide. In addition the present invention relates to the use of said peptide, polynucleotide, vector or fusion (poly)peptide, for the preparation of pharmaceutical compositions for the treatment of diseases and for the stimulation of natural killer cell (NK cell) activity.