Beta-thymosins are polypeptides involved in the regulation of actin polymerization and thymosin ß10 and ß4 have been implicated in sequestration of monomeric (G-) actin. Additionally, experimental overexpression of thymosin ß10 has been found to result in increases in F-actin bundles as well as in cell motility and spreading. We have studied the distribution of endogenously expressed thymosin ß10 in cultured human breast cancer cell lines. Both unperturbed monolayer cultures and wound-healing models were examined using double-staining for thymosin ß10 and polymerized (F-) actin. Our findings show that thymosin ß10 is expressed in all three-cancer cell lines (SK-BR-3, MCF-7 and MDA-MB-231) studied. No or little staining was detected in confluent cells, whereas strong staining occurred in semiconfluent cells and in cells populating monolayer wounds. Importantly, the distribution of staining for thymosin ß10 was inverse of staining for F-actin. These data support a physiological role for thymosin ß10 in sequestration of G-actin as well as in cancer cell motility.