Cloning pioneer Hwang Woo-suk resumed his stem cell research Monday in ending his self-imposed seclusion that began after controversy over his work erupted last month.

In what could signal his return to research, Hwang conducted a 30-minute experiment at a pig farm in Hongseong, South Chungcheong Province, operating on two sterilized pigs to implant cloned somatic cells.

His assistant Lee Byung-chun, a professor of veterinary medicine at Seoul National University, accompanied Hwang to the farm.

Several of Hwang's associates, however, said his health remains poor so that he might return to the hospital for further treatment.