Tokyo-based Meiji Co. said the milk was manufactured in March and April and shipped not long afterward. It wasn't known how much reached consumers.
Although radiation from the Fukushima nuclear plant, damaged in the March 11 earthquake and tsunami, has been detected in food in Japan, including vegetables and fish, this is the first time poisonous isotopes have been detected in baby formula, the Los Angeles Times reported Wednesday.
Government officials downplayed the significance of the discovery of radiation in the baby formula.
"There is no problem because the levels are within the government limit," Kazuhiko Tsurumi, a Health Ministry official in charge of food safety, said.
Environmentalists say they're not convinced all is well.
"This is not a development that buys confidence in the government's claim that everything is fine," Greg McNevin, a Greenpeace spokesman in Tokyo, said. "Even if the radiation levels in the formula are low, children are more at risk than are adults of getting cancer and other illnesses from radiation exposure.
"This isn't something newborns should be faced with. For them, the risk should be zero."