Growth factor receptor expression is detected by formation of a precipitate of polysome-bound receptor and growth factor on room-temperature incubation of mem­brane-free cytoplasm from a tumor cell with excess growth factor if the tumor cell contains actively synthesizing receptor capable of binding to the growth factor. Alter­natively isolated nuclei of a tumor cell are incubated at room temperature with varying concentrations of labelled growth factor in pure membrane-free cytoplasm of the tumor cell, in cytoplasm of the tumor cell con­taining cell membrane, in synthetic medium and in syn­thetic medium containing microsomal fractions from the tumor cell, so as to cause the nuclei to incorporate the labelled growth factor, and the levels of nuclear uptake in each fraction are compared.