I am trying to study for chemotherapy resistance for two drugs. I was wondering if anyone knew the exact protocol for developing a drug resistant cell line. From the articles that I read they only say that drug resistant cell lines were produced from stepwise additions of the drug of interest but there was no detailed protocol. At what confluency is the drug initially added? Do we increase the drug dosage after every day and would we double the concentration each time? Also, I'm not sure how to determine the IC50 values for the cell lines. I did the MTT cell proliferation assay using different drug dilutions and obtained absorbance values from ELISA readings. However, I'm not quite sure what to make of the values in order to calculate the IC50. I have the values for untreated and drug treated cells but I don't think that half the absorbance of the control equate to the IC50 does it? Anyone have any idea?