John Amster is on a mission: Create a more "efficient" market for IT patents while protecting mainstream tech companies from the so-called trolls, outfits that do little but buy and hold patents with the intention of suing deep-pocketed infringers. Almost two years into that endeavor, Amster's venture-backed RPX Corp. counts 50-plus member companies and has invested more than $220 million to acquire more than 1,400 patents and patent rights in a range of tech sectors.

No question, RPX is in this to make money. But unlike the trolls, also known as "non-practicing entities" or NPEs, it insists it will never assert patent rights against any member or nonmember company, so its posture is defensive, not offensive. RPX buys tech patents or rights that its management team and member companies deem to be a potential threat and then it licenses those rights to members for an annual fee--between $50,000 and $5.2 million, based on a formula tied to their operating incomes. Its two main considerations before buying a patent: Would a lawsuit based on the patent stand a chance in court? And is the patent in the hands of someone who's likely to assert it against one of RPX's members?

Started in November 2008 and backed by Kleiner Perkins, Charles River Ventures, and Index Ventures, RPX now has sufficient revenue to buy $100 million in patents and patent rights a year, making it far more financially successful than any NPE, Amster says.

RPX members are large and small companies in a range of sectors, including consumer electronics (Sony, Samsung, Panasonic, LG), mobile communications (Nokia, Verizon Wireless), e-commerce (Barnes & Noble), software (Microsoft, Symantec, Lawson), semiconductors (Intel, Nanya Technology), as well as four of the broadest industry players (IBM, Hewlett-Packard, Cisco, and Dell). Amster says companies from multiple sectors are often sued for infringing the same patent.

Amster acknowledges that RPX management will weigh the input of higher-paying members more than lower-paying ones when deciding which patents and rights to buy. However, RPX's priorities shift on a weekly basis, he says, based on the balance of its portfolio. If it has accumulated a lot of patents in, say, the e-commerce and semiconductor sectors--two areas that are heating up of late--but few in consumer electronics or mobile, it will look more aggressively to those latter areas.

Because RPX isn't trying to extract a return on the patents it buys, "we can't hold members hostage" to paying higher rates as the market value of the RPX portfolio increases, Amster maintains. So if RPX is taking specious tech patents off the market and sitting on them, what's to keep nonmembers from free-riding? For one thing, RPX sometimes just buys the license rights for clients. And sometimes it sells patents it has acquired, protecting existing members through a "vesting" program that gives them perpetual rights. Another benefit of RPX membership, according to Amster, is the "early warning" information gathered by its 55 staff experts who do nothing but monitor the secondary market for tech patents. He says no intellectual property law firm in the world offers that kind of coverage.