2-(N- Methylanilino )naphthalene-6-sulfonyl chloride (Mns-Cl) was used to label up to eight sites, presumed to be lysine residues, on rabbit skeletal muscle actin. The resultant Mns-actin was highly fluorescent, displaying emission bands near 445 and 480 nm. Incorporation of three Mns groups per actin resulted in impairment of the ability of actin to polymerize. Final relative viscosities attained for salt-induced polymers of actin having no bound Mns groups and three bound Mns groups per actin, respectively, were 1.9 and 1.4. More extensive modification further lowered the ability of actin to polymerize, but the changes were less dramatic. The fluorescence properties of Mns-actin samples that contained up to eight Mns groups per actin were not affected significantly by the addition of salt to the samples. It seems that the modified lysines either lie outside a region of direct intersubunit contact in F-actin or that the modification of a particular lysine residue or residues on a particular actin prevents that actin unit from adding on to a growing F-actin chain. Modification of actin with Mns-Cl does not affect its ability to bind and to inhibit DNase I until more than three Mns groups have been incorporated into each actin. The loss of DNase I inhibitory power only becomes apparent after the ability of actin to polymerize has been reduced substantially.