Insight into the circumstances attending the freezing and thawing of biological materials suspended in cryoprotective solutions might be expected to result from knowledge of the colligative properties and glass behavior in systems based on typical physiological media, NaCl, and important cryoprotective agents. Differential thermal analysis has been used to determine phase diagram relationships in the aqueous-rich region of the quaternary system composed of a complex physiological support medium (Eagle's minimum essential medium), NaCl, and two cryoprotective compounds (glycerol and dimethyl sulfoxide). Thermograms revealed behavior corresponding to the primary crystallization of ice from sample solutions as well as the glass transition and devitrification of nonequilibrium amorphous phases. In most quaternary solutions, equilibrium-phase formation in the form of a pseudoeutectic transformation was inhibited and the formation of a metastable amorphous phase was observed as the final mode of solidification. © 1979.