The Chelsea Kennel Club at 213 Seventh Ave., near West 22nd Street. View Full Caption DNAinfo/Maya Rajamani

CHELSEA — A “boutique” pet store that sells “quality pedigree and designer puppies” mistreated some of the dogs and hid their illnesses from customers, a new Humane Society of the United States report claims.

For two months, an investigator from the Humane Society worked undercover at the Chelsea Kennel Club at 213 Seventh Ave., near West 22nd Street, and documented what she saw with a hidden camera, the organization said.

Many of the store’s puppies — which can often be seen frolicking in its window display — had fevers, infections and other illnesses that weren’t immediately treated, according to the report. Sick puppies with medical problems were allegedly kept in an “isolation room.”

The investigator witnessed “shocking disregard” for the puppies the store sold, according to John Goodwin, senior director for the organization’s Stop Puppy Mills campaign.

“A few of the many dogs who were sick in the store include a Pomeranian whose eyes were swollen shut with conjunctivitis, a French bulldog puppy whose weight plummeted from 4.12 pounds to 3.08, an English bulldog puppy with severe pneumonia and a Shiba Inu with bronchitis,” the report claimed.

The investigator also witnessed employees “bullying” puppies by smacking them with towels, grabbing them by their scruff, and holding them down “as part of what some of the staff called ‘dominance’ training,” the report claims.

Additionally, staff often withheld files detailing “abnormal veterinary findings” from customers who bought puppies, according to the report.

“Sales staff told the investigator that the store owner had instructed them to do this in contravention of New York State’s pet lemon law, which requires pet sellers to disclose puppies’ physical abnormalities or illnesses at the time of sale,” the report said.

When the Humane Society sent “secret shoppers” into the store to buy two puppies, an employee failed to tell them that one of the dogs had been returned to the store after allegedly biting its previous owner, the report claimed.

The other puppy had been sick for weeks, but two different employees allegedly told the shoppers the dog had just gotten a cough a few days before.

The Humane Society said it passed the results of its investigation along to the mayor's office and the state attorney general's office.

A spokeswoman for the AG's office said it "has been in touch with the Humane Society with regard to their report released today and... plan[s] to look into their complaint."

The mayor's office, meanwhile, said the city's Health Department and the NYPD sent inspectors to the facility on Tuesday.

"We have zero tolerance for animal cruelty, and are investigating this facility," spokeswoman Olivia Lapeyrolerie said.

The website for the kennel touts "over 25 years of experience" for "the pet store without cages," noting in its "health care and guarantees" section that all its puppies have current vaccinations.

"Our puppies come to us with veterinarian-issued health certificates, and they are examined again by our vet and issued again with health certificates when they arrive at Chelsea Kennel Club," the site reads.

The store’s owner didn’t respond to a request for comment.