(KGO) - A lab test of ten California wines concluded they all contained trace amounts of the active ingredient from some weed-killers, glyphosate. Whether that is cause for concern is a matter of great dispute.

We know how important this issue is to so many people, so we traveled to St. Louis to see the lab that performed the tests. We also spoke to scientists, including a toxicologist from the Monsanto Company, which first patented glyphosate in the 1970's.

Wines from the prime growing regions of Napa, Sonoma and Mendocino Counties were tested at Microbe Inotech Labs in St. Louis, Missouri.

Dan Noyes: "How many of those wines had glyphosate?"
Dr. Bruce Hemming, Microbe Inotech Labs: "All 10 did."
Noyes: "All ten."
Hemming: "All ten."

Dr. Bruce Hemming and his team have been testing foods and beverages for glyphosate for years. Critics charge that these preliminary tests don't prove there's glyphosate in the wine. Doctor Hemming agrees more tests are needed, but stands by his results.

Noyes: "How confident are you that there was glyphosate in these wines?"
Hemming: "Very confident that the glyphosate is present."

Glyphosate is the most popular weed killer in the world, the active ingredient in hundreds of products, including Roundup, widely used on big farms and in home gardens. In fact, Dan Noyes found a bottle in his own supplies at home. And it's often used at vineyards.

Jennifer Putnam is the Executive Director of Napa Valley Grape Growers. She questions the results of the St. Louis tests, which were sponsored by an anti-pesticide group called "Moms Across America", but says she welcomes more testing. "Nobody's more interested in a healthy environment of the vineyard than us," Putnam said.

For decades, Roundup has enjoyed a reputation as a safe alternative to harsher chemicals, but today it's at the heart of a scientific controversy.

Roundup maker Monsanto says that over 40 years, there have been hundreds of studies on glyphosate.

"The consensus is that it is a-- will pose no unreasonable risk to human health and the environment when used according to label directions," said Monsanto Toxicologist Dr. Donna Farmer.

Dr. Farmer disputes the validity of the wine tests, but adds that even if there is glyphosate in the wine, it's way too little to cause harm.

Farmer said, "A person, say a 150-pound person would have to drink around 8,000 regularly sized bottles of wine in one day to meet the allowable daily intake for glyphosate."

The U.S. EPA says that for a 150-pound person, it's safe to consume 120 milligrams of glyphosate per day, the amount we showed in a teaspoon.