Coronavirus-like serum particles, Australia antigen, and Dane particles were treated by various chemicals and enzymes. Results were studied by electron microscopy. Coronavirus-like particles have no nucleocapsid, no detectable nucleic acid, no antigenic relationships with Australia antigen, and seem to be phospholipoproteins. Similar particles have been found in sera of guinea pigs and in human lipoproteins, especially in the HDL and VHDL fractions. Australia antigen which appears devoid of nucleic acid, and the envelope of Dane particles are sensitive to most agents and behave as phospholipoproteins. Phospholipase C produces ring-like structures. Results differ markedly from those obtained by other workers. The capsid of Dane particles is resistant to most agents. It is separated from the envelope by Tween 80, urea, and other agents. Dane particles seem to contain nucleic acid.