Prohibiting someone from making money for donating an irreplaceable kidney is one thing. But what about donating bone marrow, which replenishes itself within weeks?

That question is at the heart of a new lawsuit, filed Monday, challenging the constitutionality of the federal law that prohibits compensating bone marrow donors. The plaintiffs want to make modest recompense for such donors legal — say, paying partial tuition for a college student or making a mortgage payment for a first-time home buyer.