This subject has been discussed a number of times in Tamil Brahmin Forum. However I give below another quote from The Yakhsa Prashna (also known as the Dharma Baka Upakhyan: Story of the Righteous Crane) appears in the Vana Parva (or Aranyaka-parva or Aranya-parva) of the great epic, the Mahabharata. Yaksha is non other than Yama Dharma, father of Yudhishtira. Thus Yaksha Prashna is considered "An Encounter Between Dharma, The Father And Yudhishtira, The Son" .
Q: Yaksha: 'By what, O king, birth, behavior, study, or learning does a person become a Brahmana? Tell us with conviction!'
A: Yudhishtira: 'Listen, O Yaksha! It is neither birth, nor study, nor learning that is the cause of Brahmanahood. Without any doubt, it is behavior that constitutes it. One's behavior should always be well-guarded, especially by a Brahmana. He who maintains his conduct unimpaired, is never impaired himself. Be it a Guru or a Sishya, in fact, all who study the scriptures, if addicted to wicked habits, are to be regarded as illiterate wretches. He only is learned who performs his religious duties. He even that has studied the four Vedas is to be regarded as a wicked wretch. He who performs the Agnihotra and has his senses under control, is called a 'Brahmana!'
Ofcourse, very few will qualify to be called "Brahmana" under this definition.
Brahmanyan,
Bangalore.