Rio Olympics champion Jemima Sumgong has failed the IAAF out-of-competition drugs test.
Sumgong tested positive for the banned substance EPO, IAAF said on Thursday.
"We can confirm that an anti-doping rule violation case concerning Jemima Sumgong (Kenya) has commenced this week," the IAAF said in a statement.
"The athlete tested positive for EPO (Erythropoietin) following a no-notice test conducted in Kenya.
"This was part of an enhanced IAAF out-of-competition testing programme dedicated to elite marathon runners which is supported by the Abbott World Marathon Majors group."