These are the principal sources of the world's drug problem.

The plantation was now the family's principal source of income.

The river serves as the principal source of water for eastern Colorado.

By the 1850s he had become "the principal source of such publications in the country".

In the present day, tourism is the principal source of income.

Washington appears to be the principal source of confusion in the markets.

The principal source in the human diet is olive oil.

But last week, the principal source of these smuggled arms was cut off.

His articles and book reviews were the principal source of its success.

The principal source of 5β-coprostanol in the environment is from human wastes.