A fluoroimmunoassay for the determination of serum or plasma levels of propranolol was developed using antibodies to propranolol coupled to magnetizable solid-phase particles and fluorescein-labeled propranolol as tracer. The method was sufficiently sensitive, precise, and specific for application to routine monitoring of propranolol therapy, and gave good correlation (r = 0.99) with a widely used ultraviolet fluorometric method in the assay of patients' specimens. The fluoroimmunoassay involves the same instrumentation as the fluorometric assay and has practical advantages, including greater sensitivity (only 100 μl of sample required), avoidance of an extraction step, and visible-wavelength fluorometry, which permits the use of disposable plastic apparatus throughout the entire procedure.