The immunogenic properties of the following purified antigen preparations were studied: a detergent-soluble-lipoprotein, a phospholipase A-treated-lipoprotein, a succinylated-lipoprotein, and a low-phos-phorus-protein. The three last-named preparations were derived from the detergent-soluble-lipoprotein. All four preparations induced the formation of hemagglutinating antibodies when injected into allogeneic hosts, and sensitized allogeneic hosts against the H-2 antigens of subsequent skin grafts. The detergent-soluble-lipoprotein, the phospholipase A-treated-lipoprotein, and, to a lesser extent, the low-phosphorus-protein also sensitized allogeneic hosts against the non-H-2 antigens of subsequent skin grafts. The time-response curve following injection of detergent-soluble-lipoprotein into allogeneic hosts showed an immune peak at 5–7 days. The level of immunity waned to a low level at 14 days rising to a second peak at 56 days.