BBC Focus Magazine

[M Paul Lloyd]

Actually I think it more accurate to say that the Viking results were ambiguous rather than negative.

Carbon-rich organic molecules, which serve as the building blocks of life, may be present on Mars after all, say scientists - challenging a widely-held notion of the Red Planet as barren.
When Nasa's two Viking landers picked up and examined samples of Martian soil in 1976, scientists found no evidence for carbon-rich molecules or biology.
But after the Phoenix Mars Lander discovered the chlorine-containing chemical perchlorate in the planet's "arctic" region in 2008, scientists decided to re-visit the issue.They travelled to the Atacama Desert in Chile, where conditions are believed to be similar to those on Mars.
After mixing the soil with perchlorate and heating it, they found that the gases produced were carbon dioxide and traces of chloromethane and dichloromethane - just like the gases released by the chemical reactions after the Viking landers heated the Martian soil more than three decades ago.

http://www.bbc.co.uk/news/science-environment-11201699

[Healerman]

As I recall (I may be wrong), the "life" experiment on Viking did produce a result, the response to which was not Now that's unexpected but interesting but was instead Oh no! That can't be right! and was dismissed.

[M Paul Lloyd]

It was (if memory serves) a unexpectedly violent reaction that overwhelmed the sample chamber, not unlike pouring vinegar onto sodium bicarbonate.

It was a result, but definitley not the one they expected.