Microsoft says it will share blueprints for its Windows operating system and .NET server software with technical experts who build and maintain computer systems.
The move will provide the experts, known as system integrators, with the closely held source codes for some of Microsoft's most successful products.
To qualify, the integrators must already be high-level Microsoft support partners, meaning they build or provide technical support for companies that use its systems.
The three-year licenses are part of Microsoft's plan to let some of its best customers and partners see the inner workings of its systems.
The technical experts will see the source code via a searchable Web site that can be accessed only with a smart card.
Source codes are key to building software and represent the result of countless research dollars and hours of programmers' time.
When computer users buy software, they get a version called object code, in language understandable only by the machine. Source code is the original set of instructions, in human-readable programming language. Rivals could build better competing products if it falls into the wrong hands.
Microsoft Program Manager Jason Matusow said systems integrators will be able to use the source code to resolve privacy issues or make systems run more efficiently.
Microsoft will maintain intellectual property rights and closely guard how the source code can be used.
But some analysts said the move is a defensive strategy against so-called "open source" initiatives such as Linux, whose source code is readily available to anyone. That system has made it easy for systems integrators to build custom software.