The use of a recently developed EIA using antisera raised against purified baculovirus expressed recombinant Mexico virus (rMx) capsid protein is described for screening for human calicivirus in stools. The results show that MX-like viruses have been circulating in the UK periodically since 1983 and were an occasional cause of sporadic cases of diarrhoea in infants and outbreaks of infection among elderly patients in hospitals and old people's homes. Further evidence is presented that some strains of caliciviruses with characteristic surface morphology (HuCVs) and some with an indistinct appearance, small round structured viruses (SRSVs) are antigenically related to MxV. Tests on SRSVs from four unrelated outbreaks typed as UK3 failed to react in the Mx EIA or recombinant Norwalk virus (rNV) EIA. A 2-month survey of 206 children treated in two London hospitals for diarrhoea showed that only one was positive for MxV, a child known to be infected with HIV-1. None of the samples reacted in the rNV EIA.