Active cell death in hormone-dependent cells was studied using cultured human mammary carcinoma cells (MCF-7) treated with the anti-estrogens (AEs) tamoxifen (TAM), 4-hydroxy-tamoxifen (OH-TAM) or ICI 164 384 (10−8–10−5M) as a model. The following results were obtained. (i) In untreated MCF-7 cells a wave of replication occurred in the first 5 days of culture. All three AEs caused a dose-dependent inhibition of cell replication. (ii) TAM and OH-TAM at 10−5 M, but not ICI 164 384, caused lytic cell death (necrosis) within 24 h, which was not inhibited by estradiol (10−9–10−6M). (iii) Lower concentrations of TAM or OH-TAM (up to 10−6M) or ICI 164 384 induced a more gradual appearance of cell death beginning at day 3. This type of cell death was inhibited by estradiol (10−9 M), indicating its active nature. (iv) Nuclei showed two distinct patterns of alternation: (a) apoptosis-like condensation and fragmentation of chromatin to crescent masses abutting the nuclear envelope; (b) condensation of the chromatin to a single, pyknotic mass in the center of the nucleus, detached from the nuclear envelope. Quantitative histological evaluation revealed the predominance of pyknosis. (v) Biochemical DNA analysis revealed that only a relatively small amount of the total DNA was finally degraded into low molecular weight fragments (20 kb and less). (vi) Active cell death, with both apoptotic and pyknotic nuclear morphology, was associated with extensive formation of autophagic vacuoles (AV). 3-Methyladenine, a known inhibitor of AV formation, partially prevented cell death as detected by nuclear changes. (vii) ICI 164 384 was about 10 times more effective than TAM or OH-TAM at inhibiting DNA synthesis, but had equal potency in inducing active cell death. It is concluded that AEs have anti-proliferative and anti-survival effects on MCF-7 human mammary cancer cells in culture. These two effects are under separate control because they differ by kinetics, dose dependence and sensitivity to the various AEs. Active cell death in MCF-7 cells seems to be initiated by autophagy, in contrast to concepts of apoptosis, and thus corresponds to autophagic/lysosomal or type II death as previously defined. This may be important because of biochemical and molecular differences between these various subtypes of active cell death.