Great Britain's women's 4x400 metres team could only finish fourth at the Penn Relays in Philadelphia.
Britain's first-choice quartet of Nicola Sanders, Christine Ohuruogu, Shana Cox and Perri Shakes-Drayton were beaten by the two United States teams and one from Jamaica as they finished in three minutes 28.71 seconds.
The British quartet took gold at the World Indoor Championships in Istanbul last month and should be medal contenders at this summer's Olympics.
The USA Red team of Francena McCorory, Allyson Felix, Natasha Hastings and Sanya Richards-Ross won in 3mins 21.18secs.
Cox, Ohuruogu and Shakes-Drayton, along with Marilyn Okoro, had earlier come second in the sprint medley, also to an American team.