The first sentence of reference 15 in the report "Single-chain antigen-binding proteins" by Robert E. Bird et al. (21 Oct., p. 423) should have read, 'The majority of experiments have produced Ka's within a factor of 2 of these values; therefore, log K(a)'s for the 4-4-20 I, Fab, and 4-4-20/20' single-chain protein are 10.2, 9.9, and 9.0+/-0.3, respectively.