Alkylphenols, anthropogenic estrogenic endocrine disruptors in vertebrates, have been found in lobsters (Homarus americanus) in New England sites. We hypothesize that alkylphenols interfere in the shell hardening during molting. We used an in vitro cuticle bioassay to investigate the effects of 2 alkylphenolic compounds—2,4-bis-(dimethylbenzyl) phenol (compound 3) and bisphenol A (BPA; 4,4′-dihydroxy-2,2-diphenylpropane (also referred to as 4,4′-(propan-2-ylidene) diphenol)) on tyrosine incorporation during the hardening of new cuticle following lobster molting. During sclerotization, both alkylphenols and cold tyrosine competed with C¹⁴-tyrosine incorporation in a concentration-dependent manner. This process was also phenoloxidase dependent, as treatment with phenylthiourea (PTU; a phenoloxidase inhibitor) significantly decreased C¹⁴-tyrosine incorporation. We also found that incorporation of C¹⁴-2,4-bis-(dimethylbenzyl) phenol during the shell hardening process was inhibited by cold alkylphenol, cold tyrosine, or PTU, and competition was concentration dependent. Furthermore, incorporation of tyrosine and derivatives into new cuticle decreased with time after molting from 27% incorporation 1 day after a molt to 6% by 4 days after a molt. In nonmolting cuticles, there was no incorporation of alkylphenol or tyrosine derivatives. When lobsters were injected with 2,4-bis-(dimethylbenzyl) phenol during the premolt stage, it took the shells 12 ± 1 days to harden sufficiently to resist deflection by 5 lb pressure exerted by a pressure gauge, compared with 7 ± 1 days for control shells. Thus, shell hardening is delayed significantly by the presence of 2,4-bis-(dimethylbenzyl) phenol. The effects of this compound on shell hardening may result in lobsters' susceptibility to microbial invasion and, therefore, may contribute to the onset of shell disease.