Retroviral strains of the non-M, non-O HIV-1 group, in particular a strain designated YBF30, its fragments and also its uses as a diagnostic reagent and as an immunogenic agent.The HIV-1 viruses which differ both from the M group and the O group exhibit the following characteristics: little or no serological reactivity with regard to the proteins of the M and O groups and strong serological reactivity with regard to the proteins which are derived from the strain YBF30 according to the invention or the strain CPZGAB SIV; absence of genomic amplification when using primers from the env and gag regions of the M and O HIV-1 groups; genomic amplification in the presence of primers which are derived from the YBF30 strain according to the invention; and homology of the products of the envelope gene which is greater than 70% with regard to the YBF30 strain.