Abstract:To further clarify the mode of action of estramustine, the influence on macromolecular synthesis in the human prostatic tumour cell line 1013L was investigated. Cell treatment with estramustine, nor‐nitrogen mustard and tauromustine, followed by radioactive nucleotide and leucine incorporations, as a measure of RNA, DNA and protein labelling, were carried out. The initial effect of estramustine clearly differed from that obtained after treatment with nor‐nitrogen mustard and tauromustine. No inhibition of DNA synthesis was found whereas an inhibition of overall RNA synthesis was predominant. Adaption of an established RNA separation method was used in an indepth study of RNA labelling after estramustine treatment. An inhibition of 29S, 18S and 4–7S RNA was found after estramustine treatment, indicating disturbances in either RNA processing or RNA transport. The lack of 45S RNA labelling additionally indicates pre‐ribosomal inh