The effects of cisplatin and transplatin on the conformation and association of F-actin were studied by ESR, turbidimetry and CD measurement. The results indicate that the major component affecting F-actin secondary conformation is derived from their effect on the tertiary or quaternary conformation and association states of F-actin. The effect of platinum is seen by the fact that binding of cisplatin causes first an increase in molecular size followed by a later decrease. The induction time for the association depended on the concentration of cisplatin. Transplatin was able to reach the target site more readily, resulting in the dissociation of F-actin even at a very low concentration.