U.S. consumer groups, scientists and food companies are testing substances ranging from breakfast cereal to breast milk for residues of the world's most widely used herbicide on rising concerns over its possible links to disease.

The focus is on glyphosate, the active ingredient in Roundup.

Testing has increased in the last two years, but scientists say requests spiked after a World Health Organization research unit said last month it was classifying glyphosate as "probably carcinogenic to humans."

"The requests keep coming in," said Ben Winkler, laboratory manager at Microbe Inotech Laboratories in St. Louis.

The commercial lab has received three to four requests a week to test foods and other substances for glyphosate residues. In prior years, it received only three to four requests annually, according to its records.

"Some people want to stay out in front of this. Nobody knows what it means yet, but a lot of people are testing," said Winkler.

Microbe has handled recent requests for glyphosate residue testing from small food companies, an advocacy group testing baby formula and a group of doctors who want to test patients' urine for glyphosate residues, said Winkler. The firms and doctors do not want their identities published.

Abraxis, a Warminster, Pennsylvania-based diagnostics company, has also seen a "measurable increase" in glyphosate testing, said Abraxis partner Dave Deardorff.

Monsanto, the maker of Roundup, on April 1 posted a blog seeking to reassure consumers and others about glyphosate residues.

"According to physicians and other food safety experts, the mere presence of a chemical itself is not a human health hazard. It is the amount, or dose, that matters," Monsanto senior toxicologist Kimberly Hodge-Bell said in the blog.

Trace amounts are not unsafe, she stated.

Company spokeswoman Charla Lord said last week that further questions could be directed to the U.S. Department of Agriculture.

There are numerous studies that have determined glyphosate to be safe, but several others have linked it to human health ailments.

Critics say they fear that glyphosate is so pervasive in the environment that extended exposure even to trace amounts can be harmful.

Tests by Abraxis found glyphosate residues in 41 of 69 honey samples and in 10 of 28 soy sauces; Microbe tests detected glyphosate in three of 18 breast milk samples and in six of 40 infant formula samples.

North Dakota State University agronomist Joel Ransom reported to the U.S. Wheat Quality Council in February that tests he ordered showed traces of glyphosate in several U.S. and Canadian flour samples.