The National Institute of Standards and Technology (NIST) recently announced that an enhanced version of its CONTAM indoor air quality modeling software is now available. CONTAM 2.4b is the latest edition of the popular NIST tool that predicts airflows and contaminant concentrations in multizone building systems. CONTAM is based on a graphic interface that allows the user to draw floor plans on a "sketchpad" and employ "icons" to represent airflow paths, ventilation system components and contaminant sources. CONTAM has been used at NIST to study the indoor air quality impacts of heating, ventilation and air conditioning systems in single-family residential buildings, ventilation in large mechanically ventilated office buildings, and radon entry and transport in large residential, office and school buildings. CONTAM 2.4b may be downloaded free of charge at www.bfrl.nist.gov/IAQanalysis/software/index.htm.