genetic analysis of echovirus 11 variability in adsorption to human erythrocytes the frequency of nonhemagglutinating h mutants was determined in hemagglutinating ha clones of echovirus 11 the ha clones were derived from the prototype strain gregory and a clinical isolate of echovirus 11 the h mutants were found at a constant frequency of 30 x 105 mean value in the ha clones derived from both strains since the conditions of the population equilibrium of ha clones and neutrality of the mutation were satisfied it was proposed that the observed frequency of h mutants occurred as a result of a point mutation the frequency of h revertants at the second passage of h mutant clone was 91 x 105 hence the frequency of reversion suggests that the number of potential sites of back mutation was restricted