Antisera against acebutolol were produced in rabbits immunized by means of this drug conjugated with bovine serum albumin. These antisera were used to develop a method of radioimmunoassay for acebutolol. The plasma radioimmunoassay, described here, requires no extraction and is very easy to perform besides being quick, specific and sensitive. As little as 2.97 X 10(-9) mol/l of acebutolol can be detected. This radioimmunoassay is suitable for assaying the large number of samples usually measured in pharmacological studies.