ABC still was negotiating yesterday its share of what could be a five-year, $1 billion contract to televise major league baseball starting in 1984.

Half of the contract was settled two weeks ago by NBC, which will pay $500 million for three World Series in those five years, plus Saturday afternoon exclusivity, which means no local broadcasts except over cable or pay TV. NBC offered to pay $400 million for the other half of the contract if it was turned down by ABC, with which it shares the four-year, $190-million baseball package that ends after this season.

There were reports this week that ABC had rejected the reported $500 million asking price for the second half of the contract, which includes Monday night exclusivity and two World Series. Sources at ABC said the network agreed Wednesday night to a revised six-year pact that would give ABC a third World Series--since the series, with its relatively huge, week-long ratings, virtually pays for an entire season of usually mediocre ratings. ABC last year averaged a 4.6 rating on Sundays; NBC's '82 series averaged a 28.

"None of it is true," ABC spokesman Irv Brodsky said yesterday. "The only true statement is that we're still negotiating."

CBS, where the prime-time schedule is in need of considerably less help than at either ABC or third-place NBC, turned down baseball's $500 million offer Tuesday night. Though CBS would like a baseball contract, said CBS Sports President Neal Pilson, the network thought the price too high.