San Francisco ? Business software maker PeopleSoft Inc. turned down rival Oracle Corp.’s $26-per-share takeover bid Monday, maintaining a defiant stance likely to keep the bad blood boiling in a battle that started eight months ago.

Pleasanton, Calif.-based PeopleSoft’s eight directors reached the unanimous decision after concluding Oracle’s latest all-cash bid of $9.4 billion remains inadequate and faces a significant chance of being blocked by antitrust regulators.

It’s the third time PeopleSoft’s board has spurned Oracle since the unsolicited bidding started at $16 per share in early June. The conflict has exposed a prickly relationship between the two companies’ chief executives, Oracle’s Larry Ellison and his former subordinate, PeopleSoft’s Craig Conway, who continued to express his contempt Monday.

“Oracle’s offer does not begin to reflect the company’s real value,” Conway said in an icily worded statement. “Don’t underestimate the significant value PeopleSoft can create once the disruption from Oracle’s hostile activities has ended.”

In a brief response, Redwood Shores, Calif.-based Oracle called its bid “full and generous” and urged PeopleSoft shareholders to “act in their own best interests” by accepting the bid, despite the board’s recommendation.

PeopleSoft’s cold shoulder dashed hopes that Oracle’s sweetened bid — a 33 percent increase from its previous offer of $19.50 per share — would transform the hostile showdown into a more friendly negotiation.

PeopleSoft shares fell 53 cents Monday to $22.75 in trading on the Nasdaq Stock Market, where Oracle’s shares declined 14 cents to $13.28.

PeopleSoft’s brusque move caused some industry observers to question whether the company’s management is putting its own interests ahead of shareholders’.

“This is an offer that’s so high that it’s hard to understand the board’s rationale for not entering into a discussion over a fair price,” said Ken Marlin, an investment banker in New York who has been closely following Oracle’s bid.

“It’s clear PeopleSoft management does not want to be acquired, but this shouldn’t be about doing what’s best for them.”

Investment banker Paul Crisci of Broadview International called PeopleSoft’s rejection “stunning. It makes you wonder if this is turning more into a personal grudge where a deal isn’t getting done because the two CEOs have a disdain for each other,” said Crisci, who specializes in software industry acquisition.