FSH glycoform tracers, in the presence and absence of 2000-fold cold eFSH, were incubated with rat testis FSH receptors at 37°C for the indicated times, then placed in an ice water bath. When all the tubes were incubated, they were centrifuged to separate bound from free hormone, the supernatants aspirated and 125I-hFSH glycoform bound to the pellets counted in a gamma counter. Specific binding is plotted against incubation time. Representative data from three experiments.