In 1976 the NASA Viking landers took samples of soil on Mars and tested them for signs of organic carbon. A reinterpretation of the results now suggests the samples did contain organic compounds, but the results were not understood because of the strong oxidation effects of perchlorate, a salt now known to be found in Martian soils.
… Reinterpreting the Viking results in the light of the new findings suggests the samples from landing site 1 contained 1.5 to 6.5 ppm organic carbon, while those from landing site 2 contained 0.7 to 2.6 ppm organic carbon.
The presence of organic material does not provide evidence of life or past life on Mars but only of the presence of organic compounds. NASA is now planning a new mission for November 2011 to have another look for organics and other chemicals on Mars in an effort to better understand the chemistry of Martian soils.