SynopsisThe extraordinarily large number of immunoglobulins renders them an intriguing class of molecules for attempts to predict their conformations. The predictive method applied, using a 20 X 20 table of the observed effects of nearest-neighboring amino acids on the conformation (@,* angles) of the middle residue in known proteins, indicates positions of tripeptides that tend to break a-helices or regular 0-sheets. This 20 X 20 table is derived from data on 19 proteins, as compared with the earlier version based on 12 proteins, and includes a separate listing of residues of 0-turns that have helical +,* values. Secondary conformations predicted by methods of Chou and Fasman, Lim and Burgess, Ponnuswamy, and Scheraga have also been compared; for all three methods, wrong predictions of residues in 0-sheet conformation exceed correct ones. Better predictions are obtained when there is agreement with two or three of the methods. If there is consistent overprediction of @-structure, as with the Chou and Fasman method, the use of the 0-sheet-breaking tripeptides can improve predictability somewhat.