Abstract: A method for quantifying an amount of a viable microorganism includes subjecting a fluid sample suspected of containing a viable microorganism to a temperature change, and correlating the temperature history of the fluid sample to the amount of the viable microorganism contained in the fluid sample. The method may include the steps of bringing, the fluid sample to a first temperature, and transferring the fluid sample to a second temperature that is different than the first temperature. After the step of transferring, next is the step of measuring a temperature change in the fluid sample over a predetermined period of time. The temperature change may then be correlated to the amount of the viable microorganism contained in the fluid sample. The method finds use in a variety of applications, including evaluation of compositions or compounds potentially having microbicidal, microbiostatic, or growth enhancing properties.