SUMMARY The five vesicular stomatitis viruses Indiana, New Jersey, Cocal, Argentina and Brazil have been examined for cross-relationships by complement fixing and neutralization tests. With the exception of the Cocal and Argentina strains, intact virus particles showed little cross-reaction in either test. However, the infective skeleton-like structure produced by uncoating the virus with Tween-ether or Nonidet, and the infective ribonucleoprotein (RNP) produced by deoxycholate disruption of the virus, were neutralized by either homo- or heterotypic antisera and showed considerable cross-reactivity in the complement fixation test. Experi- ments with the isolated sub-units of the virus indicated that the major part of the cross-reactivity was associated with the RNP. The morphologically similar Piry and Chandipura viruses appeared to be unrelated to the vesicular stomatitis group in both complement fixation and neutralization tests.