  Spectrophotometry in the 3400-7400 range is presented for 13 areas of the
brightest H II region in the SMC: NGC 346. The observations were obtained at
CTIO with the 4-m telescope. Based on these observations its chemical
composition is derived. The helium and oxygen abundances by mass are given by:
Y(SMC)=0.2405+-0.0018 and O(SMC)=0.00171+-0.00025. From models and observations
of irregular and blue compact galaxies it is found that dY/dO=3.5+-0.9 and
consequently that the primordial helium abundance by mass is given by:
Yp=0.2345+-0.0026 (1-sigma). This result is compared with values derived from
Big Bang nucleosynthesis, and with other determinations of Yp.
