The intrinsic viscosity ([η]) and the molecular weight (M) by sedimentation equilibrium were determined for hyaluronic acids of low (M=104−7.2×104) and high (M=3.1×105−1.5×106) molecular weights. Double logarithmic plot of [η] against M gave different lines for the two groups. The relationship between [η] and M was [η] =3.0×10−6 × M1.20 for the former and [η)=5.7×10−4×M0.76 for the latter group. The molecular weight at the point of intersection of the two lines was about 1.5×105. The rheological behavior of the hyaluronic acids below M=2.1×104, for which the value of reduced viscosity was independent of concentration, was different from that of the hyaluronic acids above M= 5.7×104, for which the value of reduced viscosity increased with concentration.