A radioimmunoassay for the direct measurement of carteolol, a new beta-adrenoreceptor blocker, in human plasma was developed. Carteolol was acylated to form O-glutarylcarteolol, which was conjugated to bovine serum albumin to provide the immunogen. Antibody to carteolol was raised in New Zealand albino rabbits. The tracer was the radioiodinated derivative of O-glutarylcarteolol-tyrosine methyl ester conjugate. The method is highly sensitive, with a lower quantifiable concentration of approximately 0.4 ng of carteolol/ml using 0.1 ml of plasma, and has good specificity, with the major metabolite (8-hydroxycarteolol) showing only 0.2% cross-reactivity. It is reproducible, with relative standard deviations from triplicate standard curves being mostly within +/- 8%. The method is currently being used to monitor carteolol levels in clinical samples.