New measurements of the lithium abundance in a sample of 37 field dwarfs of low metallicity (-0.6 ≥ [Fe/H] ≥ 3.2) are presented. Combining these observations with those in the literature it is shown that extremely metal-deficient ("EMD") dwarfs ([Fe/H] ≤ -1.4) form a much more homogeneous population with regard to 7Li abundance than stars somewhat more metal-rich. The authors consider that these stars offer the best opportunity to make a reliable measurement of the primordial Li abundance, which they estimate to be log N(Li) = 2.2(±0.15), a value slightly higher than that given by Spite and Spite (1982). Using a recent theoretical curve of Li against the universal baryon to photon ratio, η, from a standard Big Bang model, and the above cited primordial abundance, η is constrained to the range 1.2×10-10 ≤ η ≤ 8×10-10.