Growth rate, morphology, and responsiveness to mitogenic stimuli and pharmacological treatments were evaluated in early and late cell passages derived from the same clone of the widely used MCF-7 human breast adenocarcinoma cell line. Our results indicate dissimilarities between early (E) and late (L) passages for some of the parameters analyzed. The cells that underwent many subcultivations grew faster than the others; both appeared homogeneous in size and shape. The E cells, subcultured for almost 1 yr, displayed higher sensitivity to the mitogenic action of both estradiol, according to the level of estrogen receptor, and insulin-like growth factor-I than did the L cells, kept in culture for more than 10 yr. Cell responsiveness to two drugs, a novel steroid antiestrogen and a polysulfonated distamycin A derivative, was more pronounced in the early cultures only at the longer time of exposure to the higher concentration of the estrogen antagonist. In addition, a drug-induced inhibition of insulin-like growth factor-I binding to its receptor was shown in both E and L cells, the latter being less sensitive than the former when exposed to the antiestrogen. Finally, MCF-7 E and L cells showed similar behavior when drug-induced apoptosis was tested.