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SUMMARY: Methods are described for the purification of Murray Valley encephalitis virus from baby mouse brain extract. The method finally chosen involved treatment with protamine, ultracentrifugation, adsorption to and elution from hydroxyl apatite and a second ultracentrifugation. The degrees of recovery of virus infectivity and haemagglutinin were about 15 % and 50 %, respectively. Measurement of the absorption in the ultraviolet region and examination in the electron microscope suggested that a substantial degree of purification had been achieved. The stability of the infectivity of purified virus was less than that of crude virus. Purified virus yielded infective ‘ribonucleic acid’ on treatment with phenol. The yield was consistent with the possibility that the potential infective ‘ribonucleic acid’ in crude virus was contained in infective virus particles.