What is the penalty for not enough players on the football field?

In American football, there is no penalty for not enough players on the field for a play. Should there be too many players on the field, there is a penalty called for illegal participation (five yards). But if a team has too few, there is no penalty called. There technically is no penalty for having too few players on the field. However, the offense always has to have at least seven men on the line of scrimmage and an eligible receiver on each end of the line. If the offense didn't meet this requirement, they would be flagged for an illegal formation.