Serological cross-reactions among eighteen virus isolates of the tombusvirus group were compared in precipitin tube and immunodiffusion serological tests. The isolates were also compared by immunoelectrophoresis in agar gel. Although precipitin tube tests showed considerable and reproducible differences between the various isolates, the results were too greatly affected by other factors to be of value in assessing strain relationships. When pairs of isolates were compared for spur formation in gel-diffusion tests, the results suggested that most isolates could be placed in one of two groups ; one group comprised isolates from pelargonium (leaf curl), the other consisted of petunia asteroid mosaic virus and artichoke mottled crinkle virus isolates from Italy and tomato bushy stunt isolates from soil around this Institute and from cherry. Four isolates did not fall into either of these groups ; they nearly always formed spurs when compared among themselves, or with viruses in either of the two groups. Pairs of isolates that could be distinguished from each other in spur-formation tests using antiserum homologous to one of them could nut always be differentiated when antiserum heterologous to both isolates was used. Immunoelectrophoresis gave consistent results with several methods of virus preparation; it indicated grouping and separation of the isolates in general agreement with the results of gel-diffusion tests: all pelargonium leaf curl isolates were grouped together with slow migration towards the cathode. The petunia asteroid mosaic isolate and the isolates from cherry and from soil from this Institute (GCRI) moved slowly towards the anode. Tomato bushy stunt virus type strain migrated rapidly to the cathode, differing greatly from all other isolates. The method offers a relatively simple means of typing isolates of the tombusvirus group.