  A new determination of the pregalactic helium abundance based on the
Magellanic Clouds H II regions is discussed. This determination amounts to Yp =
0.2345 +- 0.0030 and is compared with those derived from giant extragalactic H
II regions in systems with extremely low heavy elements content. It is
suggested that the higher primordial value derived by other authors from giant
H II region complexes could be due to two systematic effects: the presence of
neutral hydrogen inside the helium Stromgren sphere and the presence of
temperature variations inside the observed volume.
