A new assay for amphotericin B has been devised with use of the release of a radioactive potassium analogue (86rubidium; 86Rb) from Candida parapsilosis. The 86Rb-release assay is rapid, highly sensitive, and reproducible. Amphotericin B levels of 0.06-2.0 mug/ml can be measured in serum, whereas as little as 0.015 mug of amphotericin B/ml can be detected in spinal fluid. The greater sensitivity of the assay in spinal fluid as compared with that in serum may be the result of binding of the amphotericin molecules by serum components, thus decreasing the amount of amphotericin free to interact with the fungal membrane.