Thymosin alpha 1 and thymosin beta 4 are two thymosin fraction 5-derived peptides with the capacity to alter a variety of immune functions in human and animal models. In this study we investigated the effect of both thymosin alpha 1 and thymosin beta 4 on human colonic lamina propria lymphocyte (LPL) proliferation and ornithine decarboxylase (ODC) activity. LPL from eighteen human colon specimens were cultured in the presence or absence of thymosin alpha 1 and thymosin beta 4. We found that both peptides suppressed thymidine incorporation into LPL. However, thymosin alpha 1 and thymosin beta 4 did not alter thymidine incorporation into phorbol ester (PDB) and calcium ionophore (ionomycin)-stimulated LPL. Furthermore, thymosin alpha 1 and thymosin beta 4 also did not alter ODC activity in Con A-stimulated LPL. These results suggest that both peptides alter LPL proliferation, and that the mechanism for this inhibition may not involve the calcium fluxes or the ODC pathway but may involve protein kinase C. We postulate that thymosin alpha 1 and thymosin beta 4 may participate in the modulation of the human mucosal immune system.