UUP leader Mike Nesbitt has challenged Upper Bann MP David Simpson to show him "evidence" of dirty tricks during the election campaign.
The DUP man launched an outspoken attack on his rivals during his acceptance speech in Banbridge on Friday morning.
He hit out at "despicable attacks" on his family from unionist quarters and said the campaign had "stooped to the gutter".
It is thought the Co Armagh man, who overcame the challenge the UUP's Jo-Anne Dobson by more than 2,000 votes, was referring to comments made on social media referring to his three adopted children.
In an open letter to Mr Simpson, Mike Nesbitt said last night he took the allegations against members of his party "most seriously".
"As I hope you are aware, I have in the past demonstrated zero tolerance to members of my party who act in unacceptable ways, whether it be expressions of homophobia, or public displays of disloyalty," he said.
"If any member of the Ulster Unionist Party has indeed acted in the manner you appear to allege, I assure you I shall not be found wanting in my reaction, but first I need the evidence.
"I would have preferred to correspond with you privately, but as you have put your allegations into the public domain, the choice of communication platform was already chosen and this is therefore, an open letter."
Congratulating Mr Simpson on his victory, Mr Nesbitt also asked him to tell him the name of the company who conducted a poll published in the Portadown Times which suggested Sinn Fein's Catherine Seeley was the DUP's closest challenger for the Upper Bann seat.
She eventually finished 4,000 votes behind in third.
Ms Dobson insisted following Mr Simpson's speech that her campaign had been "totally positive", and complained of DUP scare tactics warning of a unionist split would allow Sinn Féin in.
One of Mr Simpson's children, Leah Cassells, who works for her father in his parliamentary office and is a Harvard graduate in law and international business, took to Facebook last week in response to the online comments.
She and her twin brother Steven were adopted from Paraguay while sister Kristy is originally from India.
"God blessed us with wonderful, Godly parents and we are so proud to be able to call them mum and dad," Leah said.
"It is with sadness that I read certain posts from UUP supporters over the past few days about our adoption. Comments that should never be made so flippantly about adoption of children.
"We love our parents as much as they love us. We will not let online bullies get us down, we feel sorry for them."
Mr Simpson could not be contacted for comment last night.
