The UK Anti-Doping agency says it sent two investigators to Kenya to look into allegations that four British athletes were doping with the blood-booster EPO in a well-known high-altitude training region.
Ukad (UK Anti-Doping) was responding to allegations made by German broadcaster ARD and the Sunday Times, who said three Kenyan men told them in an undercover sting that they had either provided or administered EPO to the British athletes.
No athletes were named.
Ukad chief executive Nicole Sapstead says the investigators "are currently in Kenya pursuing a number of lines of enquiry."
Ms Sapstead said: "UK Anti-Doping has reviewed the evidence presented to us by the Sunday Times and it is of grave concern and of significant interest."