The sequences of myocardial excitation in the cases of human LBBB have been studied by the surface mapping technique. Analysis of the surface map patterns of 12 subjects has enabled 4 different types to be described. It is suggested that Type I represents a complete LBBB in the presence of an intact right bundle branch, while Types II and III show different degrees of conduction defect in the right bundle branch, too. The left ventricular activation of these 3 types has a similar pathway in phases II and III which is determined by orientation of the main subepicardial muscle layer coursing through the apex and terminating in the anterobasal region. The terminal anterobasal activation in the cases of LBBB has not only theoretical, but clinical significance, since in the presence of anteroseptal myocardial infarction the signs of necrosis must be hidden not in the early, but in the terminal parts of the QRS complex of conventional ECG. The lack of terminal anterior positivity of surface maps in the case of LBBB may be indicative of chronic anterior myocardial infarction.