Abstract
Spleen cells from mice infected with scrapie virus were separated into subpopulations on the basis of buoyant density in discontinuous gradients of isotonic albumin or differential adherence of cells to plastic. At three different intervals after infection, a population of "less dense" cells was found in albumin gradients that had 40-to 60-fold higher specific infectivity (cells per median lethal dose) than the total cell suspension before gradient sedimentation. The class of cells associated with high relative specific infectivity has a density characteristic of lymphoblasts, myeloblasts, and macrophages. Separation of "macrophage rich" cells on the basis of adherence to plastic did not result in significant enrichment of scrapie virus-infected cells.