An 11-month-old boy was given a bottle feared to contain bleach as he sat with his mother and grandmother in a fast food restaurant.
The toddler swallowed some of the contents and immediately vomited, Scotland Yard said. He was taken to hospital for observation and his condition is stable.
Police are looking for a white woman in her fifties who gave the child the bottle in McDonald's, Eltham High Street, south east London.
The restaurant is temporarily closed and officers are looking at CCTV footage of what happened. They have recovered the bottle and tests are being carried out to find out what it contained.
A spokeswoman for London Ambulance Service said: "We were called just before 12.40 today to reports of an incident in Eltham High Street, and sent one crew.
"Our staff treated a baby boy who had been taken unwell, who was taken to Queen Elizabeth Hospital, Woolwich."
The Metropolitan Police alerted the public to the danger via Twitter. In a message from @MPSGreenwich, police said: "WARNING! Last 1/2 hr. #Eltham High St. White female in 50s giving children in pushchairs bottles containing believed bleach."
Staff at McDonald's on Eltham High Street refused to comment.
The woman is heavily built with grey hair, and was wearing black trousers and black trainers with a long multi-coloured top and a black jacket with white chevron-type markings. She was carrying a maroon bag.
Police believe only one child was a victim. Anyone with information can call the incident room on 020 8284 9543 or Crimestoppers on 0800 555 111.