The application of reverse transcription-polymerase chain reaction (RT-PCR) using primers directed to the RNA dependent RNA polymerase region within ORF1 of Norwalk virus (NV) showed that 31 percent of morphologically typical human caliciviruses (HuCV) and 57% of small round structured viruses (SRSVs) produced a product of 470 bp similar to the NV control, NV 8FIIa/68/US. Alignment of the amino acid sequences of morphologically typical HuCVs with previously published sequences for SRSVs, NV, and Snow Mountain agent (SMA) showed a high degree of homology (90-92%) with SMA and a lesser extent of homology with NV (60-61%). The amino acid sequence of two strains of HuCV, HuCV/3C/92/UK, and HuCV/5C/92/UK differed by only one or two amino acids respectively in the RNA dependent RNA polymerase region from that of two strains of SRSV obtained from children in the United Kingdom, SRSV/4S/90/UK and Japan, SRSV/OTH-25/89/J which were found to have identical amino acid sequences. The use of an EIA for detection of NV antigen employing antisera raised to recombinant NV protein indicated that HuCVs and SRSVs obtained from children and adults in the United Kingdom were antigenically distinct from the prototype Norwalk virus, NV/8fIIa/68/US.