An antigen recognized by antisera produced against p30 (core) proteins of the four chief groups of mammalian type C viruses (murine, feline, RD-114 related to endogenous primate, and infectious primate group) is located in an immune-complex pattern in some renal glomeruli of human SLE patients with lupus proliferative glomerulonephritis but is not detected in normal or pathological control human kidneys. This antigen cross-reacts with p30 interspecies determinants shared by the four chief virus groups and cross-reacts with a partially purified antigen extracted from human SLE spleen. The human SLE spleen antigen cross-reacts with p30 group antigen of RD-114 virus but not of feline or murine viruses. Some host immunoglobulins eluted from a human SLE kidney by acid-buffer show antibody-like activity against p30 group antigen of RD-114 virus but not of simian, feline, or murine viruses.