The stickers are valid for any of the new environmental zones
Three German cities - Berlin, Cologne and Hanover - have introduced "environmental zones" to reduce fine particle emissions from traffic.
Drivers now have to display a coloured sticker on their vehicle to enter the inner city zones. The colour depends on the pollutants the vehicle emits.
The cities are gradually phasing in fines of 40 euros (£29;$58) for anyone caught driving without a sticker.
Other German cities - but not all - plan to have such zones later in 2008.
The stickers - green, red or yellow - are mandatory not only for locals but also for foreign drivers, including tourists.
There is a one-off charge of five to 10 euros for the stickers, issued by Germany's vehicle registration authority and authorised garages.
Some hotel and restaurant owners have voiced fears that tourists will be put off by the requirement for stickers, reports in Germany say.
The German motoring club, the ADAC, plans to take legal action against the restrictions, the DPA news agency reports.
The EU has set the limit for fine particle pollutants at 50 micrograms per cubic metre of air, which cannot be exceeded on more than 35 days per year.
The Berlin environmental zone covers about 88sq km (34sq miles), while the one in Cologne covers about 16sq km.
Officials say the majority of cars in the affected cities qualify for the stickers.