Carlos Danel and Carlos Labarthe turned a nonprofit that lent money to Mexico’s poor into one of the country’s most profitable banks. But not all of their colleagues in the world of microlending approve of this. Many microlending advocates, including the economist Muhammad Yunus, founder of the Grameen Bank, and winner of the 2006 Nobel Peace Prize, say that success of microfinance is threatened by this market-oriented model, with its emphasis on investor returns.

“Microfinance started in the 1970s with a focus on using this breakthrough to help end poverty,” said Sam Daley-Harris, director of the Microcredit Summit Campaign, a nonprofit endeavor that promotes microfinance for families earning less than $1 a day. “Now it is in great danger of being how well the investors and the microfinance institutions are doing and not about ending poverty.” He said the situation posed the danger of “mission drift.”

Mr. Danel and Mr. Labarthe say microfinance will help more poor people by tapping investor capital rather than the limited pool of donor money. The bank began as a NGO in 1990, started by a Catholic social action group, whose inspiration was a visit by Mother Teresa to Mexico.