After months of off-and-on electronic war with America Online over "instant messaging" on the Internet, Microsoft Corp. has beaten a retreat.

Microsoft has decided to stop distributing software that attempts to let users send instant messages over the Internet to people who use AOL's popular messaging software. AOL has been blocking the messages since Microsoft first released its software in July, contending that they intruded into its data network and jeopardized the online security of people who use AOL's service.

Although Microsoft responded by releasing new versions of its software that attempted to get around AOL's defenses, AOL moved quickly to foil them. For weeks, two of the world's largest technology companies traded digital thrusts and parries.

After developing at least two dozen versions of its MSN Messenger software, Microsoft said it was giving up because AOL's blocking efforts were creating "very serious" computer security risks for users of the Microsoft software.

Instant messages appear on a recipient's computer screen almost as soon as they are sent and allow "real-time" typed communication among people who are on the Internet. To trade instant messages, though, both sender and recipient typically have to use the same software.

The fast-growing instant-messaging market has been dominated by AOL, which pioneered the technology on its proprietary online service and later made the software available free to any Internet user. According to AOL, more than 45 million people use its messaging software, making it the de facto standard.

Microsoft had intended for MSN Messenger, which has 4.5 million users, to crack AOL's dominance by allowing people to reach not only Microsoft users but also people who use AOL's software.

The move raised the ire of AOL, which accused Microsoft of an "unauthorized intrusion" into its network. Microsoft said it was only trying to promote "inter-operability" between different messaging systems.

Microsoft insisted yesterday that it had not lost the high-stakes messaging war.

"Nobody has won here," said Deanna Sanford, a lead product manager in Microsoft's Internet division. "Consumers want inter-operability, and they won't get that until there is an instant-messaging standard that allows them to communicate with one another regardless of which instant-messaging application that they choose to use."

Sanford said Microsoft changed course because of a "huge security risk" posed by AOL's recent blocking tactics.

An AOL spokeswoman said the Dulles-based firm was pleased that Microsoft was "respecting the privacy and security of our instant-message users."

Both firms have pledged to work on developing a common system for people to trade instant messages. Most of that work is being done by a group set up by the Internet Engineering Task Force, a consortium of technology businesses and specialists.

AOL has voiced concerns that hasty development of a common standard could create many of the problems Internet users have with e-mail today, particularly the transmission of electronic viruses and a flood of unsolicited messages, commonly known as "spam."

Nevertheless, AOL executives vowed in July to "work closely" with the task force. The company also announced that an advisory committee of industry executives it was forming would collaborate with the panel, lending a business perspective to the task force's largely technical discussions.

But the co-chair of the IETF task force, AT&T Corp. researcher Vijay Saraswat, said yesterday that AOL has not been an active participant in the group's discussions. Although the company sent a representative to the task force's last meeting, he said AOL employees have not regularly engaged in the group's online discussions, where much of its work takes place.

"Most of our stuff happens online, and hitherto, I have not seen very strong participation from the people at AOL," Saraswat said.

The task force includes people from major technology companies such as Microsoft, AT&T, International Business Machines Corp. and Lucent Technologies Inc. He said the group is "very, very keen for increased participation from AOL."

AOL spokeswoman Ann Brackbill insisted that the company has "actively been supporting the IETF."

"We actively monitor the online discussions each day and we look forward to the next meeting," Brackbill said.

She also said that members of AOL's advisory committee--which includes such high-profile industry leaders as Apple Computer Inc. interim chief executive Steve Jobs, Real Networks Inc. Chairman Rob Glaser and Novell Inc. chief executive Eric Schmidt--have had at least one phone conversation since the group was formed in July.