The inactivation of foot-and-mouth disease virus by formaldehyde was studied under different conditions, both as free virus and (as in routine vaccine production) after adsorption of the virus to aluminium hydroxide gel (alhydrogel). In the latter case infectivity was monitored after elution of the virus from the gel by isopycnic ultracentrifugation of the virus-alhydrogel mixture in CsCl. By this method good virus recoveries were obtained. Adsorption of the virus to alhydrogel (without formaldehyde) did not reduce infectivity significantly. Both adsorbed and non-absorbed virus lost infectivity at a rate of about one log10 per day (at pH 8.5, 25 degrees C--no formaldehyde). Kinetics of formaldehyde inactivation of adsorbed and non-adsorbed virus were also identical, with a fast reduction in the initial phase (in case of O1 and A10-virus approximately one log10/hour). After this initial phase inactivation became linear and rather slow (for O1 and A10-virus 0.2 log10/hour). No "tailing-off" was observed. Under standard conditions (0.04 per cent formaldehyde, pH 8.5, 25 degrees C) CD-virus was inactivated approximately 1.5 times faster than O1 and A10-virus. At 4 degrees C the inactivation of the three strains continued at about one log10/day. Increased lactalbumin hydrolysate concentrations reduced the inactivation rate, especially at the formaldehyde concentration of 0.02 per cent, which was originally applied. Quaternary amines like Tris strongly inhibited formaldehyde activity. These findings might explain some data of others who observed "tailing off". Analysis of formaldehyde inactivated antigen by SDS-PAGE and electrofocusing showed that extensive cross-linking occurs especially of VP1, probably with other virus proteins but also with non-virus proteins from the medium. VP2 and VP3 are less affected. Cross-linking was enhanced when the virus had been adsorbed to alhydrogel during inactivation. Progressive cross-linking was observed during storage of the vaccine at 4 degrees C, which also indicated that inactivation continued at this temperature. These data show that formaldehyde inactivated adsorbate vaccines can be safe.