A letter from the underwriters (or initial purchasers, in the case of a Rule 144A offering) to the issuer confirming the information contained in the prospectus (or offering memorandum) that was supplied by the underwriters (or initial purchasers). The blood letter is delivered in connection with the indemnification provisions of the underwriting agreement (or purchase agreement) in which the underwriters (or initial purchasers) agree to indemnify the issuer for losses arising out of any misstatements or omissions arising out of information that they have supplied for the registration statement or any prospectus (or the offering memorandum). Typically, the letter refers only to information included in the "Underwriting" (or "Plan of Distribution") section of the prospectus (or offering memorandum).

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For a form of blood letter, including integrated notes with important explanations and drafting tips, see Standard Document, Blood Letter.