Uber is bringing a small number of self-driving cars to its ride-hailing service in San Francisco.
The move is likely to excite the city's tech-savvy population and certain to antagonise California regulators.
The launch in Uber's home town expands a public pilot programme the company started in Pittsburgh in September.
The testing lets everyday people experience the cars as Uber works to identify glitches before expanding the technology's use in San Francisco and elsewhere.
California law, however, requires a test permit for self-driving prototype vehicles, and Uber does not have one.
The company argues that the law does not apply because its cars require a human backup.
Uber has a history of testing legal boundaries. Although the company has been around less than a decade, it has argued with authorities around the world about how much of its drivers' histories should be covered in background checks and whether those drivers should be treated as contractors ineligible for employee benefits.
Uber's self-driving tests in San Francisco will begin with a "handful" of Volvo luxury SUVs - the company would not release an exact number - that have been tricked out with sensors so they can steer, accelerate and brake, and even decide to change lanes.
The cars will have an Uber employee behind the wheel to take over should the technology fail.
Users of the app may be matched with a self-driving car, but can opt out if they prefer a human driver. Self-driven rides cost the same as ordinary ones.
The cars will be put to the test in the congested streets of San Francisco. The city can be a daunting place to drive given its famously steep hills, frequent fog, street and cable cars, an active bicycle culture - and roads that are constantly being repaved, remarked and restricted for bike lanes and traffic management.
Uber believes its technology is ready to handle all this safely, though its executives concede the vehicles are nowhere near able to drive without a human ready to take control in dicey situations.