Identification and quantitation of cellular proteins associated with HIV-1 particles are complicated by the presence of nonvirion-associated cellular proteins that copurify with virions. Many cellular proteins are associated with nonviral particles that bud from the surface of cells called microvesicles. Microvesicles band in sucrose gradients in a range of densities that includes the same density as retroviruses. To characterize these microvesicles, HIV-1-infected and uninfected human T-cell lines were propagated and virus and microvesicles were purified from clarified cell culture supernatants by sucrose density gradient centrifugation or centrifugation through 20% sucrose pads. Microvesicles were found to contain various proteins, including HLA DR and β2-M, and a substantial amount of RNA and DNA. The concentrations of HIV-1 p24^CA, HLA DR, and β2-microglobulin (β2-M) were determined by radioimmunoassay. The ratios of HIV-1 p24^CAto HLA DR and β2-M were found to vary with respect to the HIV-1 isolate, host cell, and other factors. Electron microscopic analysis of microvesicles revealed that they consisted of particles of various sizes and morphologies. Although HIV-1 particles are known to contain some cellular proteins, microvesicles from HIV-1 infected H9 cells appeared to contain little or no HIV-1 gp120^SU.