The efficacy of sodium hypochlorite was assessed against human immunodeficiency virus type 1 suspended in low (8% v/v) or high (80% v/v) concentrations of serum or in a high (80%) concentration of blood. In the presence of 8% serum. 100 p.p.m. available chlorine in the disinfectant test mixture inactivated 3·75 log TCID50 HIV/ml within 30 s. When the test mixture contained 80% serum. 500 p.p.m. available chlorine inactivated more than 4 log TCID50 HIV/ml in 1–2 min. Lower concentrations of available chlorine were unable to inactivate the virus completely. In the presence of 80% blood. 1000 p.p.m. available chlorine in the disinfectant test mixture was unable to inactivate 3·75 log TCID50 HIV/ml. although 2500 p.p.m. available chlorine was able to inactivate at least 1·5 log TCID50 HIV/ml. In all test mixtures, the chlorine rapidly became combined and thus less active. Our results emphasise the importance of cleaning prior to disinfection with sodium hypochlorite since it may prove to be ineffective in the presence of high levels of organic matter. In cases where prior cleaning is impossible, care must be taken to use the higher recommended concentration (a minimum of 10000 p.p.m. available chlorine).