Rubella virus (RV) strains vary in their abilities to replicate and persist in cell cultures derived from human joint tissue (synovial cells [SC]), and this arthrotropism appears to be linked to their association with joint symptoms in vivo. In order to map the genetic determinants of arthrotropism, an infectious clone of the Cendehill vaccine strain of RV was constructed, as well as two chimeric clones containing cDNAs from both Cendehill and Therien (wild-type) strains. Replacement of the entire structural gene region of Therien in the infectious clone pROBO302 with the corresponding region of Cendehill did not affect growth in SC. A further observation that Cendehill bound equally well to SC and the permissive Vero cell line indicated that restriction was not at the level of receptor binding, a function of the envelope proteins. Mutations that affected growth in joint cells were mapped to two locations in the nonstructural gene region. The first of these (nucleotides 2803 and 6416) resulted in a 10-fold decrease in yield of progeny virus from SC. This region contained five mutations, at nucleotides 2829, 3060, 3164, and 3528 (near the carboxy terminus of P150 where the protease domain is located) and at nucleotide 4350 in p90. Further substitution of the sequence representing nucleotides 1 to 2803 to give a complete Cendehill infectious clone restricted growth in SC by a further 100-fold to less than 10 PFU/ml. This region contains three mutations, at nucleotides 34, 37, and 55, within the 5' stem-loop structure. In conclusion, the Cendehill-specific mutations believed to be determinants of joint cell growth are located in two regions, the 5' nontranslated region and in a sequence that encodes the carboxy-terminal region of p150 extending into the helicase domain of p90.