A peptide fragment or a series of peptide fragments containing one or more selenocysteine that has a lowered toxicity than selenocystine and that exhibits a cytotoxicity-inhibitory activity. The peptide fragment or a series of peptide fragments according to the present invention has preferably the amino acid sequence from 260th to 362nd amino acid residues from the C-terminal of selenoprotein P, or said amino acid sequence with one or several amino acid residues therein being deleted, substituted or added, or a partial sequence of either of the above amino acid sequences, or an amino acid sequence comprising as a part any of the above amino acid sequences. A screening method for a peptide fragment having the cytotoxicity-inhibitory activity is also provided.