Human papovavirus, RFV, isolated from urine of a renal transplant patient was compared with two strains of SV40 and with the prototype human papovavirus, BKV. Neutralization tests showed that RFV and BKV are indistinguishable, while large-plaque (LP) and small-plaque (SP) isolates of SV40 gave a low but significant level of cross-reaction with rabbit or human antisera against RFV.