Dependence of melting temperature Tm on guanine + cytosine (G + C) content of various samples of DNA obtained from different organisms. DNA was dissolved in 0.15 M NaCl + 0.015 M Na-citrate, pH 7.0. Points 1 and 41, for poly(dA-dT) and poly(dG-dC), fall off the least-squares line which is described analytically by Tm = 69.3 + 0.41 (%GC).