Twenty-two fragments of β-actin and β-actin-related protein were isolated from the acidic extracts of rat spleen tissue. β-Actin fragments (75-90), (78-89), and (78-88), 0.01-1 μM, decreased live cell number of L929 murine tumor fibroblasts by 80-90%, with maximal cytotoxic effect of 30-40%. The fragments of (78-90) segment and the fragment of β-actin-related protein (69-77) were less active (inhibitory effect up to 55%, cytotoxic-up to 25%). Copyright © 2008 European Peptide Society and John Wiley & Sons, Ltd.