SAN FRANCISCO -

A co-owner of a Northern California slaughterhouse accused of processing cows with cancer has pleaded guilty to a criminal charge.

The San Francisco Chronicle reports that 77-year-old Robert Singleton, co-owner of Petaluma-based Rancho Feeding Corp., entered the plea on Friday to aiding and abetting in the distribution of adulterated, misbranded and uninspected meat. He has agreed to work with prosecutors who have filed charges against the company's other owner, Jesse Amaral Jr., and two employees, Eugene Corda and Felix Cabrera.

They have pleaded not guilty.

Prosecutors say the company slaughtered dozens of cows with skin cancer of the eye, and plant workers swapped the heads of diseased cattle with those of healthy cows.

Operations were halted in February after a series of recalls, including one for 8.7 million pounds of beef.