1. Considerable differences were found between the rates and degrees of modification of native trout actin with iodo[2-14C]acetate and iodo[1-14C]acetamide. 2. With iodoacetate, G- and F-actin were both labelled in the N-terminal peptide only. This modification had little effect on the ability of the actin to polymerize. 3. Iodoacetamide labelled three cysteine residues in both G- and F-actin. The modified cysteine residues were identified from the position of the corresponding tryptic peptides on peptide `maps'. 4. The modification had little effect on the ability of G-actin to polymerize, to bind ATP or to bind Ca2+, or on the ability of F-actin to depolymerize. 5. It is concluded that the three cysteine residues present on the `surface' of the native trout actin molecule have no direct role in the polymerization processes, the binding of ATP, or the binding of Ca2+.