Blood lactate is a clinically valuable diagnostic indicator. In this preliminary report we describe a protein biosensor for (L)-lactate based on beef heart lactate dehydrogenase (LDH). LDH was noncovalently labeled with 8- anilino-1-naphthalene sulfonic acid (ANS). The ANS-labeled LDH displayed an approximately 40% decrease in emission intensity upon binding lactate. This decrease can be used to measure the lactate concentration. The ANS-labeled LDH was further utilized in a new sensing format, polarization sensing, which is suitable for miniaturization to a point-of-care lactate monitor. However, temporal instability of beef heart LDH indicates the need for further protein engineering prior to development of a more robust lactate-sensing protein. (C) 2000 Academic Press.