In the search for peptides that could effectively enhance the monoclonal antibody production of a model hybridoma, the performance of five lysine-containing peptides was compared. The capacity of the peptides to enhance the monoclonal antibody yield correlated with their growth-suppressing activity. No correlation of the production-enhancing activity with the character of the distribution of cell-cycle phases could be found. All of the tested peptides, including the negative control peptide Gly-Phe-Gly, altered the cell-cycle phases distribution in favor of the proportion of the S phase. The peptides added to the hybridoma culture were found to be gradually decomposed into dipeptides and free amino acids. Among the set of tested lysine-containing di- to pentapeptides, the best results were obtained with the tripeptide Gly-Lys-Gly. The growth-suppressing and production-enhancing capacity of this peptide supplement was obviously associated with the temporary presence of the intact peptide molecule in the culture media, because the addition of a mixture of free amino acids constituting this peptide, i.e., glycine and lysine, displayed a different effect-a slight promotion of cell growth.