This instrument, developed for quantitating fluorescence polarization immunoassays, automatically measures both polarization components and computes a polarization value corrected for background and optical bias. An electronically switched liquid crystal provides a nonmechanical means of rotating the plane of polarization in the excitation optics. The electronic design features digital integration and microprocessor controlled functions. Readings are made in disposable 12 X 75 mm round culture tubes. in less than 10 s. Results are precise to 0.001 polarization unit, linear from 10(-7) to 10(-10) mol of fluorescein per liter, and correlate well with those obtained with other "high-performance" instrumentation.