SAN FRANCISCO  In response to growing pressure from international law enforcement agencies and conservation groups, eBay, the online auction giant, announced Monday that it would ban all commerce in ivory, including most heirlooms, to avoid providing a market that will encourage the slaughter of endangered elephants.

The announcement, made to the company’s merchants and customers, came as a conservation organization based in Massachusetts prepared to issue the latest in a series of reports documenting how online auction sites, particularly eBay, have become a magnet for trading in items derived from endangered species, among them rare birds and reptiles sold to collectors, ivory-handled walking sticks or bracelets and figurines carved from elephant tusks.

The report, to be released here on Tuesday by the International Fund for Animal Welfare, analyzes data gathered in a six-week survey that tracked more than 7,000 listings of wildlife or their feathers, teeth or pelts offered for sale on more than 185 Web sites in 11 countries. Nearly three-quarters of the items were elephant products, the report said.

The vast majority of the online trade in endangered animals, the report says, is done on eBay. Law enforcement officials and specialists in illegal wildlife trade said it was impossible to determine how much of the estimated $10 billion spent each year in the illicit trade happens online.