TT virus (TTV) has not yet been cultured or visualized. We attempted to recover and visualize TTV-associated particles from the serum samples and feces of infected humans. Serum samples were obtained from 7 human immunodeficiency virus (HIV)-infected patients. Three patients had a high TTV DNA titer (10(8) copies/ml), three had a low TTV DNA titer (10(2) copies/ ml), and one was negative for TTV DNA. Fecal supernatant was obtained from a different TTV-infected subject. The serum samples were fractionated by highperformance liquid chromatography, and TTV DNA-rich fractions were subjected to floatation ultracentrifugation in cesium chloride. Virus-like particles, 30-32 nm in diameter, mere found in the 1.31-1.33 g/cm(3) fractions from each of the three serum samples with high TTV DNA titer, but not in any fraction from the four serum samples that either were negative for TTV DNA or had low TTV DNA titer. The TTV particles formed aggregates of various sizes, and immunogold electron microscopy showed that they mere bound to human immunoglobulin G. Similar virus-like particles with a diameter of 30-32 nm banding at 1.34-1.35 g/cm(3) mere visualized in fecal supernatant with TTV genotype 1a by immune electron microscopy using human plasma containing TTV genotype 1a-specific antibody.