Human IgG antibody subclasses have been measured in the sera of workers exposed to rats, using a crude extract of rat urinary protein antigens, in an ELISA system. The antibody titres in individuals either with or without specific IgE were similar, with the exception of IgG4 where the mean level of this subclass was lower in those individuals with measurable titres of IgE (p less than 0.01). Symptomatic individuals, with specific IgE, also had lower titres of IgG4 than the corresponding asymptomatic, IgE-positive subjects (p less than 0.05). The frequency of positives in each subclass assay was similar in both groups. These findings suggest that higher levels of IgG4 may have a protective rôle.