An 11-month-old boy who drank from a bottle of liquid given to him by a stranger is being treated in hospital.
Police say he is in a stable condition, but parents are being warned to be vigilant.
The baby was with his parents at McDonald's in Eltham, south east London, when a woman approached and gave him the bottle.
It is believed he drank the liquid and immediately vomited.
An ambulance was called and the baby was taken to Queen Elizabeth Hospital in 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."
Police later referred to the liquid as an "unknown substance".
It is thought the parents were eating at a table in the restaurant on Thursday lunchtime with the child in a pushchair next to them.
Officers have been at the scene gathering CCTV in a bid to trace the woman, who is described as heavily built with white hair and aged in her 50s.