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With the aim of developing a new series of steroidal affinity labels of the estrogen receptor, six electrophilic 11β-ethyl (C2), 11β-butyl (C4), or 11β-decyl (C10) derivatives of estradiol bearing an 11β-terminal electrophilic functionality, i.e. bromine (C4), (methylsulfonyl)oxy (C2 and C4), bromoacetamido (C2 and C4), and (p-tolylsulfonyl)oxy (C10), were synthesized. The range of their affinity constants for binding the estrogen receptor was 0.4−37% that of estradiol; the order of increasing affinity (i) relative to the 11β-alkyl arm was ethyl < butyl and (ii) relative to the electrophilic functionality was bromoacetamido < bromine < (methylsulfonyl)oxy. Regardless of the conditions used, including prolonged exposure of the receptor to various pH levels (7−9) and temperatures (0−25 °C), the extent of receptor affinity labeling by the 11β-ethyl and 11β-butyl compounds, if any, was under 10%. This was in sharp contrast to results obtained using 11β-((tosyloxy)decyl)estradiol which labeled from 60% to 90% of the receptor hormone-binding sites with an EC50 of ~10 nM. Estrogenic and antiestrogenic activities of the compounds were determined using the MVLN cell line, which was established from the estrogen-responsive mammary tumor MCF-7 cells by stable transfection of a recombinant estrogen-responsive luciferase gene. The two 11β-ethyl compounds were mainly estrogenic, whereas the three 11β-butyl and the 11β-decyl compounds essentially showed antiestrogenic activity. The fact that the chemical reactivities of 11β-ethyl and 11β-butyl compounds were not compromised by interaction with the estrogen receptor made the synthesized high-affinity compounds potential cytotoxic agents which might be able to exert either (i) a specific action on estrogen-regulated genes or (ii) a more general action in estrogen-target cells. Therefore the ability of the compounds (1) to irreversibly abolish estrogen-dependent expression of the luciferase gene and (2) to affect the proliferation of MVLN cells were determined. All electrophiles were able to irreversibly suppress expression of the luciferase gene; the antiestrogenic electrophiles were more potent than the estrogenic ones but less efficient than 4-hydroxytamoxifen, a classical and chemically inert triphenylethylene antiestrogen. Only the antiestrogenic electrophiles decreased cell proliferation; however, they were less potent than 4-hydroxytamoxifen. In conclusion, the synthesized electrophilic estradiol 11β-ethyl and 11β-butyl derivatives (i) were not efficient affinity labels of the estrogen receptor and (ii) did not display significant cytotoxicity in estrogen-sensitive mammary tumor cells. However, since these derivatives displayed high affinity for the estrogen receptor, they could be used to prepare potential cytotoxic agents which might be selective for tumors affecting estrogen-target tissues, by coupling them with a toxic moiety.