The Authors present the actual knowledge about the hepatitis B associated antigen (HBsAg) and the now more used methodics for detection of its surface component (HBsAg); consequently they present the results of a screening executed between january 1976 and december 1979, directed to research the HBsAg into sera of 655 patients affected by various diseases, particularly liver diseases, using the reverse haemoagglutination test. With this technique, that is of very easy and rapid performance, together with a good sensibility (even though it gives a rather important deal of feigned positivity, yet these can be voidable by means of an adsorption test) the Authors have totally acquired a 8.7% of positive cases for HBsAg (with a majority of patients affected by liver diseases) and a 1.2% chronic carriers for this antigen.