An IgM concentrate was purified from Cohn fraction III. Efficiency of euglobin precipitation was shown to be controlled by pH and ionic strength. Prekallikrein activator activity in the product was insignificant. Overall yield from the octanoic acid supernate and purity of the concentrate were 66 +/- 8 (n = 16) and 50 +/- 5% (n = 16), respectively. Solvent-detergent treatment to inactivate lipid-enveloped viruses was demonstrated and implemented into the process. Process studies to control residual virucidal agents and C4a generating activity are presented.