The Prostate 32:74 (1997) Response to Letter to the Editor We agree with Dr. Alexander’s suggestion that the integrity of synthetic peptides needs to be carefully ascertained for in vitro immunization studies. The synthetic peptide used in our recently published study  was confirmed to be a 9-mer peptide by mass spectroscopy performed by the manufacturer (Research Genetics, Birmingham, AL). The same peptide corresponding to residues 146–154 of prostate specific antigen (PSA146–154) has been synthesized on four subsequent occasions over a two year period. Each preparation has been confirmed by mass spectroscopy and has mediated specific lysis by the original (CTL) line. We are examining whether the PSA 146–154 peptide is naturally processed and presented for CTL recognition by tumor cells that endogenously synthesize PSA protein. It is interesting to note that Correale et. al. recently reported that an HLA-A2-restricted CTL line elicited with an overlapping peptide corresponding to residues 141–150 of PSA specifically lyses cells of the HLA-A2 phenotype that endogenously produce PSA © 1997 Wiley-Liss, Inc. protein . The PSA 141–150 peptide shares the potential to form disulfide bridges via the side chain of the cystine residue at position 149, yet appears to emulate a naturally processed epitope from PSA protein. David J. Peace, MD Loyola University Medical Center Cardinal Bernardin Cancer Center Division of Hematology/Oncology 2160 South First Avenue Maywood, IL 60153 REFERENCES 1. Xue BH, Zhang Y, Sosman JA, Peace DJ: Induction of human cytotoxic T lymphocytes specific for prostate-specific antgen. Prostate 1997; 30:73–78. 2. Correale P, Walmsley K, Nieroda C, Zaremba S, Zhu M, Schlom J, Tsang KY: In vitro generation of human cytotoxic T lymphocytes specific for peptides derived from prostate-specific antigen. J Natl Cancer Institute 1997; 89:293–300.