Plaintiff filed an adversary action seeking to have the court determine the validity, priority, and extent of its security interest in debtor's accounts. Trustee moved for summary judgment on the ground that plaintiff had failed to file a proof of claim. The court denied trustee's motion, concluding that 11 U.S.C. § 501(a) did not mandate the filing of a proof of claim, except where creditor seeks to share in a liquidation distribution. Where creditor asserts a lien on estate assets, neither § 501 nor Bankruptcy Rule 302(e) bars its claim.