Particles of tomato golden mosaic virus (TGMV), purified by an improved procedure, were used to prepare an antiserum which in gel double-diffusion tests with homologous virus gave a single precipitin line and had a titre of 1/256. TGMV was shown to be serologically related to another geminivirus, cassava latent virus, and distantly related to a leafhopper-transmitted geminivirus, beet curly top virus. No relationship could be detected to four other leafhopper-transmitted geminiviruses.