The optical brightener preferred for use with the modacrylic fiber has the general formula: ##STR5## where X and Y are basic groups selected from the group consisting of --NH2, --NH(CH3)2, and --NH+ (CH3)3 Z, where Z is selected from the group consisting of chloride, sulphate and bromide anions, n is 0 to 3, and R is an alkyl group having 1 to 4 carbon atoms.