OCT fluctuation spectroscopy provides a quantitative and non-invasive tool to monitor 3D mammary epithelial cell (MEC) models which recapitulate features of breast cancer in vivo. Cellular dynamic fluctuation measurements allow for assessing drug responses due to different mechanisms of inhibition in 3D models in vitro. Here we report on pre-malignant MEC responses to two toxicants: Taxol, which stabilizes microtubules against depolymerization, and has emerged as an important chemotherapeutic agent in the treatment of breast cancer, and blebbistatin, a small molecule inhibitor with high affinity and selectivity toward Myosin II. Here we propose a quantitative method for measuring toxicant responses of MECs in 3D cultures from OCT speckle fluctuation spectroscopy. OCT fluctuation spectra were quantified within the 9-440 mHz band, from which the inverse power-law exponent (α) and the fractional modulation amplitude (M) were extracted. Pre-malignant MECs treated with Taxol (0, 10 μM and 20 μM) exhibit an increase (p<0.001) in α along time (t=0, 1 hour, 24 hours, 48 hours and 6 days), indicating selective inhibition of high frequency fluctuations, with a concomitant decrease (p<0.001) in M over time. However, only M of pre-malignant MECs treated with Blebbistatin (0, 25 uM and 50 uM) exhibited a decrease (p<0.001) along time, but no significant change (p>0.05) in α, since myosin II isn’t highly expressed in non-migratory pre-malignant MECs. We demonstrated the feasibility of OCT fluctuation spectroscopy to quantify MEC toxicant response. Microtubule stabilization was found to be a major contributor to pre-malignant MEC fluctuation signals, while myosin II inhibition was not.