"I am a Bihari and Modi has insulted and humiliated all Biharis by questioning my DNA and using such a language," Nitish Kumar said at a press meet here.
Addressing a public meeting in Muzaffarpur, about 70 km from here, on Saturday Modi said: "There seems to be some problem in his DNA because the DNA of democracy is not like that. In democracy, you give respect even to your political rivals."
Nitish Kumar said he failed to understand how a prime minister, which is a top constitutional post, can use such language against a chief minister like him.
"It has exposed that standards have deteriorated to such an extent," he said.
Nitish Kumar said earlier senior BJP leader Nitin Gadkari said casteism was in the DNA of Bihar.
"I would like to point out to BJP leaders that they have no knowledge of Bihar's DNA. They should study Bihar's DNA, which has a long history," Nitish Kumar said.
He said people of Bihar would give a reply to Modi and BJP in the upcoming assembly polls for this insult.
Nitish Kumar used a Hindi saying "Ek Bihari Sab Per Bhari" (a Bihari is enough to handle 1,000 people) to point out that it would be proven right in the upcoming Bihar assembly polls.
"Modi and his party have no leader or face to challenge me, a Bihari, so they have borrowed leaders from outside the state for it," he said.
Nitish Kumar wondered how Modi was asking people to give him a chance to serve them. "People had voted for him and provided him a chance to serve in last Lok Sabha polls in 2014."
Kumar said now it appears that Modi "as PM is seeking additional charge of chief minister of Bihar. It is surprising and strange," he added.