The total synthesis of 5,10,15,20-tetra[3,5-(carboranylmethyl)phenyl]porphyrins 2-5 containing 36-43% boron by weight are reported. All compounds were characterized by spectroscopic methods and, in the case of 2, by X-ray crystallography. The water-soluble nido-carboranylporphyrin 5 (H(2)OCP) was found to have low dark toxicity toward V79 lung fibroblasts (CS(50) > or = 250 microM), to be readily taken up by human glioblastoma T98G cells in culture and to localize subcellularly preferentially in the cell lysosomes. In comparison with a known tetra(nido-carboranyl)porphyrin (6), H(2)OCP (5) is taken up slower and to a lower extent by T98G cells, possibly as a result of its higher hydrophilic character. The metal-free H(2)OCP (5) was also found to accumulate to a higher extent in T98G cells compared with its zinc(II) complex analog 4. Our studies show that carboranylporphyrins bearing eight nido-carborane cages can still accumulate intracellularly and have low dark toxicity toward cells in culture, and therefore might have promise for application in BNCT.