Arabinitol is a pentitol generated in large quantitites by several species of Candida, including Candida albicans. The level of arabinitol in the sera of infected animals and humans was determined by gas-liquid chromatography of an acetone extract of the serum. Experimentally infected mice with pyelonephritis due to C. albicans had elevated levels of arabinitol; rabbits with pyelonephritis did not have elevated levels, nor did rabbits with catheter-induce cystitis, but rabbits with endocarditis developed elevated levels of arabinitol shortly before death. A prospective study in patients clinically suspected of having invasive candidiasis failed to show elevated levels of arabinitol in most. Mice and patients not colonized or infected with yeasts but with renal failure had high serum levels of arabinitol. The data indicate that an elevated level of arabinitol in the serum of a patient without renal disease is suggestive of invasive candidiasis, but normal serum levels do not contradict the diagnosis.