Searches by the Viking orbiters for chlorophyll did not detect it at the scale that would have been visible on the Earth from orbit, but it still could exist at low amounts on the surface.
The Viking landers also did not detect any organics molecules at their landing sites. But recent research suggests the detectors sent may not have been sensitive to low numbers of microbes:

Vikings would have missed life on Mars
Early Mars lander missions lacked the acuity to sniff out life.
13 February 2001
Nature Science Update
"The Viking space missions to Mars in the 1970s sent back a mixed message about the red planet. The views were fabulous; but the nightlife was a little quiet. The two Viking lander craft touched down on the martian surface in 1976, scooped up handfuls of dust, and pronounced it devoid of the organic material that might signify the presence of microorganisms.
"But even if there had been several million bacteria-like cells in every gram of Martian soil, the Viking landers would not have detected them, Jeffrey Bada and his co-workers at the Scripps Institution of Oceanography in La Jolla, California, now claim1."
http://www.nature.com/nsu/010215/010215-11.html

Bob Clark