Arabinitol concentrations were determined in 157 serum samples from 95 patients with suspected invasive candidosis and in 10 serum samples from healthy laboratory workers. Fifty eight of the 95 patients, subsequently diagnosed as not having invasive candidosis had concentrations of arabinitol below 1.2 micrograms/ml (mean 0.59 (SD) 0.26). Sera from the healthy laboratory workers gave similar results (mean 0.55 (0.05]. Concentrations above the normal range were found in 18 of the 19 cases of confirmed or probable invasive candidosis and in seven of eight patients with infected intravenous lines or cannulas and clinical evidence of systemic infection. Raised concentrations were also seen in 10 other patients, including nine with renal failure who did not have invasive infections. Multiple serum samples obtained from 33 patients showed that sequential estimations were of value for diagnosing a developing infection. Despite some difficulties of interpretation the technique is rapid and specific and is suitable for use in the diagnostic laboratory of a larger general hospital.