An ELISA procedure has been developed to measure specific IgG antibodies against rat urine. Using this method antibody levels in individuals with differing exposure levels to rats have been measured. Mean antibody titres in groups of workers regularly exposed to rats were higher than those in groups currently or totally non-exposed. However, only in the group of workers exposed and sensitized to rats were IgG levels significantly elevated. Despite this significant difference in mean antibody titres there was considerable overlap between individuals in each group, and there was no direct relationship between exposure to rats and levels of IgG anti-rat urine antibody. Our results suggest a stronger correlation between sensitization to rats and the levels of antigen-specific IgG antibodies.