1. The potassium channel beta-subunit from rat brain, Kv beta 1.1, is known to induce inactivation of the delayed rectifier channel Kv1.1 and Kv1.4 delta 1-110. 2. Kv beta 1.1 was co-expressed in Xenopus oocytes with various other potassium channel alpha-subunits. Kv beta 1.1 induced inactivation in members of the Kv1 subfamily with the exception of Kv 1.6; no inactivation of Kv 2.1, Kv 3.4 delta 2-28 and Kv4.1 channels could be observed. 3. The second member of the beta-subunit subfamily, Kv beta 2, had a shorter N-terminal end, accelerated inactivation of the A-type channel Kv 1.4, but did not induce inactivation when co-expressed with delayed rectifiers of the Kv1 channel family. 4. To test whether this subunit co-assembles with Kv alpha-subunits, the N-terminal inactivating domains of Kv beta 1.1 and Kv beta 3 were spliced to the N-terminus of Kv beta 2. The chimaeric beta-subunits (beta 1/ beta 2 and beta 3/ beta 2) induced fast inactivation of several Kv1 channels, indicating that Kv beta 2 associates with these alpha-subunits. No inactivation was induced in Kv 1.3, Kv 1.6, Kv2.1 and Kv3.4 delta 2-28 channels. 5. Kv beta 2 caused a voltage shift in the activation threshold of Kv1.5 of about -10 mV, indicating a putative physiological role. Kv beta 2 had a smaller effect on Kv 1.1 channels. 6. Kv beta 2 accelerated the activation time course of Kv1.5 but had no marked effect on channel deactivation.