Catheter blockage in patients receiving long-term parenteral nutrition with fat-containing total nutrient admixture (TNA) is a relatively common complication. A study was carried out to characterize the material which is filtered out of the TNA and is a potential cause of catheter blockage. A total of 45 bags containing the same TNA solution were stored for 7 days at 4°C. The stability of the solution in all the bags was then confirmed by light microscope and Coulter Counter, to determine the particle-size distribution, following which the solution was filtered through a 5-μm filter. Chemical analysis was then made to determine the amount of solid particles, fat and precipitates of Mg, Zn, Ca, Na, and K in the filter contents. Each bag was found to contain 7326 ± 2681 solid particles as plasticizers and the main component of the filter contents was fat (99.4%) whereas electrolytes as precipitates constituted less than 0.5%. The amount of fat and electrolytes lost on the filter from the solution was negligible. Our analysis of the material trapped on the filter, which may block the catheter during long-term therapy, suggests the importance of filtration and of finding a means for dissolving the fat, the main component of the filter material.