The amount of particulate matter present in Bretschneider's cardioplegic solution (HTK) was assessed by laser-mediated particle counting. Permissible levels of contaminant particles with a distribution of diameters between 0.2 and 20 microns were found. A significant further increase in the particle count was observed when the fluid was administered for clinical use, which resulted in the additional release of particles from, for example, the infusion kit, which included an in-line filter with pores of 270 microns. Filtration of the HTK solution by a terminal inline filter (0.2 micron) significantly reduced the number of particles. In order to determine the chemical composition and the potential hazards of the particulate material we used scanning electron microscopy in combination with energy dispersive X-ray analysis and transmission electron microscopy to examine specimens taken from heart tissue obtained from Göttinger minipigs after cardioplegia and from humans undergoing mitral valve replacement after cardioplegia and reperfusion. Particles of various diameters were found either to be plugging coronary capillaries, to be adherent to the endothelial layer, or to be engulfed by polymorphonuclear (PMN) granulocytes, which appeared to be activated. Some of the PMN granulocytes were apparent in the endothelial layer. It is recommended, therefore, that a terminal in-line filter (0.2 micron) should be routinely used.