Wis. Stat. § 113.04
Subject to s. 113.03, the obligee's release or discharge of one or more of several obligors, or of one or more of joint, or of joint and several obligors shall not discharge co-obligors, against whom the obligee in writing and as part of the same transaction as the release or discharge, expressly reserves the obligee's rights; and in the absence of such reservation of such rights shall discharge co-obligors only to the extent provided in s. 113.05.
Wis. Stat. § 113.04
Release of one tort-feasor, reserving all rights against another, limits recovery against the other to the percentage of that other's negligence. Peiffer v. Allstate Insurance Co. 51 Wis. 2d 329, 187 N.W.2d 182 (1971). There is a rebuttable presumption that a general release of the original tort-feasor does not release a malpractice cause of action for damages that might be recovered against the original tort-feasor unless clearly and expressly stated in the release. Rights need not be reserved against the doctor. Krenz v. Medical Protective Co. 57 Wis. 2d 387, 204 N.W.2d 663 (1973); Westphal v. Cantwell-Peterson Clinic, 57 Wis. 2d 402, 204 N.W.2d 491 (1973).