The antimicrobial properties of gaseous horseradish distillate, composed of ca. 90% allyl isothiocyanate (AIT) and 10% $\beta$-phenethyl isothiocyanate ($\beta$-PEIT), were evaluated in agar and roast beef model systems. Growth of Listeria monocytogenes, Salmonella typhimurium, Escherichia coli, Staphylococcus aureus and Serratia grimesii was completely inhibited on agar by 2 000 nL horseradish distillate/L air at 12$\sp\circ$C and 35$\sp\circ$C. Lactobacillus sake was more resistant to the distillate and 20 000 nL/L was required for complete inhibition at 12$\sp\circ$C. Bactericidal activity varied between genera and increased with distillate concentration for each temperature. In the roast beef model system, 20 000 nL/L was required for complete inhibition of bacteria at 12$\sp\circ$C while Lactobacillus sake was only weakly inhibited at this concentration. Bactericidal activity in the roast beef model system was observed against Escherichia coli, Staphylococcus aureus and Serratia grimesii with 20 000 nL/L at 12$\sp\circ$C. Total aerobic, enteric and pseudomonad counts on roast beef were also suppressed by 20 000 nL/L at 4$\sp\circ$C over 28 d, however, lactic acid bacteria were resistant to the distillate. Depletion of residual AIT was observed n both model systems over time. Oxygen did not contribute to depletion while the presence of agar discs slightly increased depletion of residual AIT. The aroma and flavour of roast beef were significantly affected by vaporized horseradish distillate. The microbiological results suggest that atmospheres containing vaporized horseradish distillate could enhance the safety and shelf-life of pre-cooked, uncured roast beef. (Abstract shortened by UMI.)