The sandstone of cathedrals and monuments and the concrete walls of the new Elbtunnel (tunnel under the Elbe, Hamburg) were contaminated with endolithic nitrifying bacteria and their metabolic end products nitrite and nitrate. On the surface of sandstone with carbonate and silicate as binding material low amounts of nitrite and high concentratons of nitrate were measured. Nitrification was detectable from pH 4.3 to 9.0. In the range between pH 5.6 and 9.0 high activities of nitrifiers correlated with high amounts of nitrate. On the concrete surface pH values between seven and 13 were measured. Nitrification could not be detected above pH 9.5. At the values between pH 9.0 and 13 the concentrations of nitrite were high and reached the same level as the nitrate concentrations. The high amounts of nitrite and nitrate originate from NO_x emission by traffic. -- AATA