Drug efficacy assays with microplate readers are a key aspect of the modern drug discovery process. However, microtitre plate readers are expensive laboratory equipment and not easily transportable. We have created a prototype Android smartphone application that enables smartphones to measure the concentration values in microplate wells. We find that the smartphone camera measures concentrations of red and yellow solutions more accurately than green. Further, concentration readings are most accurate when microplates are backlit and sources of noise (such as glare and shadows) are removed. Therefore, we designed a simple dark box to control ambient light, which reduces the error in measurements to within 7% of a laboratory microplate reader. An affordable and mobile alternative to a microplate reader is expected to support drug research in developing countries by enabling rapid, cheap drug assays in small laboratories and in field work with a standard Android smartphone.