Gram-negative particles, found in ‘normal’ urine by an improved microscopic technique, become excessive (often > 105/ml) in many systemic diseases. In these diseases they are accompanied by sparse, usually fastidious, gram-positive cocci. Antibiotics at moderate doses usually have little effect. Larger doses of antibiotics suppress, or temporarily eliminate, the particles. In this report, the particles are characterized by light microscopy for better identification. Then, by detection of muramic acid in a hydrolysate and by transmission electron microscopy, they are identified as decomposed (‘exploded’) gram-positive cocci. Since explodeds cannot be external contaminants, and their precursors cannot proliferate sufficiently in urine, they must have crossed renal membranes to come from within the body. They are demonstrated in tissue fluids and in synovial fluids by optical microscopy, by their muramic acid, and by transmission electron microscopy. Thus, they cross other membranes. Explodeds are excessive in several rheumatic diseases, in renal diseases, in diseases in which a coccal cause has been sought, and in some in which cocci have never been considered. There is no precedent for explodeds. Their appearance and numbers are compatible with the literature on natural and experimental systemic streptococcal diseases and with the experimental illnesses following injection of streptococcal cell walls. Urinary explodeds are likely to be the end result of the ‘almost physiological’ entry of streptococci into the circulation which necessitates predental antibiotic prophylaxis in mitral disease. Increased numbers of urinary explodeds probably represent excessive entry of precursors or proliferation of precursors within the host. Urinary explodeds serve as a marker for diverse systemic diseases, systemic coc