An automatic system to monitor temperature and humidity in biological exposure chambers has been developed. The sensing of humidity is based on the psychrometric principle and monitors 24 chambers, each fitted with two thermometers (thermistors) acting as dry and wet bulbs. The temperature of the dry bulb has a range of 10-43 degrees C with a linearity better than 0-1%. Each chamber is sampled for 2 1/2 min and the temperatures are converted into proportional voltages which are fed into a microprocessor. This controls the necessary steps to calculate the relative humidity from the two temperatures. The range of the measured humidity extends from 14 to 100% with a 3-4% maximum error. The calculations are performed on a mathematical model that relates humidity to dry and wet temperatures in degrees Fahrenheit or Celsius measured at a given pressure. The results are displayed in a digital form and are also printed on a strip chart recorder. The print-out shows the chamber number being monitored, the dry-bulb temperature and the relative humidity of the respective chamber.