We have developed a microimmunofluorescence test (IF) which uses cells infected with a recombinant baculovirus which expresses the capsid proteins of astrovirus types 1 or 6. The IF test was sensitive and specific and the results for human astrovirus type 1 (HAst-1) were comparable to those obtained by immune electronmicroscopy and radioimmunoassay. Application of the test to a panel of 273 sera collected from patients and staff at two childrens hospitals in London showed that over 50% of the population were infected by HAst-1 between the age of 5 and 12 months rising to 90% by 5 years, whereas human astrovirus type 6 (HAst6) was relatively uncommon (10-30%) in all age groups.