Bozhidar Lukarski, one of the Reformist Bloc's leaders, has been cautious about future coalitions, saying the bloc should await the final results.
He described the figures as "expected", since the bloc had worked hard to do well at the elections.
Lukarski ruled out a three-party coalition if RB is in the government.
Many polls had predicted center-right GERB and the Reformist Bloc, a right-wing coalition, would be able to count on an independent majority to form a cabinet on their own.
The RB came fourth according to Alpha Research's exit poll, the first unveiled after the ban on preliminary results was lifted at 19:00 EEST.