IC50, the half-maximal inhibitory concentration, is the concentration of a compound that produces half of its maximal inhibition of a response in a given assay. It is usually estimated as the midpoint parameter of a four-parameter logistic curve fitted to response against concentration, and it depends on the assay conditions.
Why it matters when licensing company data
EC50 is the same measure for a stimulating effect. Guidelines distinguish a relative IC50, the midpoint between the fitted plateaus, from an absolute IC50 tied to the 50% control; reports should say which one they give. pIC50 is the negative base-10 logarithm of the IC50 in molar units. Because an IC50 changes with substrate concentration, incubation time and cell line, values from different assays should not be compared directly.
An IC50 can only be checked if the plate layout, raw reads, controls and fit settings are kept with it. Years of dose-response plates with that context, including failed runs, are among the lab records that describe how compounds actually behaved.
See how to calculate IC50, with a worked example and assay data at biotechs, CROs and labs.
Example
A discovery team runs an eight-point dilution of a kinase inhibitor in triplicate on a 96-well plate, with vehicle and full-inhibition controls. The analyst normalizes each well, fits a four-parameter curve and reports a relative IC50 of 0.25 micromolar with a Hill slope of 1.1. When the biotech later describes its archive, it counts how many plates keep the raw reads and layout alongside the fitted values.
Source
Sebaugh JL (2011), Guidelines for accurate EC50/IC50 estimation (PubMed). Definition reviewed November 9, 2026 by Origin Data Partners.