α-crystallin can be isolated in two forms depending on the temperature at which the lens is extracted. At 4°C,αC-crystallin is obtained while at 37°C, a smaller molecule,αm-crystallin can be isolated.The apparent molecular weight of bovine foetalαm-andαC-crystallins were determined in 5 different ways using sedimentation velocity, sedimentation equilibrium and intensity fluctuation spectroscopy analyses and the experimentally determined diffusion coefficients, intrinsic viscosity and partial specific volume. Values ranged from 291,000 to 369,000 forαmand from 604,000 to 760,000 forαCdue to the differential effects of the protein's polydispersity on the different methods.Subfractionation of the protein by gel filtration yielded much less polydisperse minimum species populations with molecular weights of 280,000 and 529,000 forαmandαCrespectively.It was concluded thatα-crystallin is probably synthesized as a symmetrical dodecamer and that the polydispersity of most preparations can be attributed to age-related modificationin vivoas well asin vitrosupra-aggregation due to variations in experimental conditions.