The Supreme Court recently held in United States v. Mead Corp. that agency interpretations should receive Chevron deference only when Congress has delegated power to the agency to make rules with the force of law and the agency has rendered its interpretation in the exercise of that power The first step of this inquiry is difficult to apply to interpretations adopted through rulemaking, because often rulemaking grants authorize the agency to make "such rules and regulations as are necessary to carry out the provisions of this chapter" or words to that effect, without specifying whether "rules and regulations" encompasses rules that have the force of law, or includes only procedural and interpretive rules. Mead therefore requires that courts decipher the meaning of facially ambiguous rulemaking grants. This Article argues that throughout most of the Progressive and New Deal eras, Congress followed a convention for signaling when an otherwise ambiguous rulemaking grant was intended to confer delegated authority to make rules with the force of law. Under this convention, rulemaking grants coupled with a statutory provision imposing sanctions on those who violate the rules were understood to authorize rules with the force of law; rulemaking grants not coupled with any provision for sanctions were understood to authorize only interpretive and procedural rules. Although this understanding can be detected in the Administrative Procedure Act of i946 (APA), the Supreme Court's decisions construing rulemaking grants after the adoption of the APA betray no awareness of the convention. In the I970s and early i98os, the D.C. Circuit and Second Circuit, in an effort to encourage greater use of rulemaking, adopted in place of the convention a presumption that facially ambiguous rulemaking grants always authorize rules with the force of law. As a result, courts held that some agencies, such as the FTC, FDA, and NLRB, had legislative rulemaking powers that Congress almost certainly had not intended. Because the Supreme Court has never endorsed the presumption of the D.C. and Second Circuits, it is not constrained in the aftermath of Mead from drawing upon the original convention in discerning whether Congress intended to delegate power to make rules with the force of law. Strong arguments exist in favor of adopting the convention as a general canon for interpreting facially ambiguous rulemaking grants. Compared to the current approach that treats all rulemaking grants as presumptively authorizing legislative rules, the convention is generally more faithful to congressional intent and to constitutional values associated with the nondelegation doctrine. These advantages, however, must be weighed against the fact that adopting such a canon at this late date would almost certainly upset reliance interests, most prominently in the FDA context.