Lugol's solution destroys the biological activities of Newcastle disease virus (NDV) after 15 s incubation at 37 degrees C. The rates of inactivation are slower at lower temperatures and at acid pH. At 4 degrees C and pH 5.8, the functions associated with the virus membrane, (haemolysis (HL), cell fusion and infectivity) are inactivated within 32 min, while haemagglutination (HA) and neuraminidase (N) are resistant to inactivation for several hours. Adjustment of NDV, Sendai and influenza A virus allantoic harvests to pH 5.8 and subsequent treatment with undiluted Lugol's solution (pH 5.8) for 15 min has a minimal effect on HA but results in complete loss of infectivity. It is suggested that iodination could be a useful method for vaccine production with membrane-bound viruses. It is postulated that the separation and dissociation of the membrane-associated properties of paramyxoviruses from the glycoprotein functions is due to the higher affinity of iodine for the lipids. Iodine could react with the carbon-carbon double bond (C=C) of the unsaturated fatty acids. This could lead to a change in the physical properties of the lipids and membrane immobilization.