A method for evaluating antibody neutralisation of HIV-1 comprising: (a) incubating a plurality of first cells each comprising at least one HIV-1 expression vector which lacks a nucleic acid encoding an HIV-1 viral envelope polypeptide and a nucleic acid encoding an HIV-1 viral envelope polypeptide sequence obtained from a patient, the patient HIV-1 envelope polypeptide sequence comprising extracellular and transmembrane domains of the viral envelope protein, wherein the patient HIV-1 envelope polypeptide sequence does or does not include the intracellular cytoplasmic tail domain or a portion thereof, and wherein the HIV-1 expression vector may comprise nucleic acid sequences encoding a constant predefined cytoplasmic tail domain; such that the plurality of first cells produce viral particles comprising a population of extracellular and transmembrane domains of the viral envelope protein from an HIV-1 virus population from the patient; (b) contacting a portion of the viral particles produced in step (a) with an HIV-1 antibody; (c) contacting the viral particles of steps (a) and (b) separately with a plurality of second cells, wherein the second cells express a cell surface receptor to which the viral particles bind and wherein either the second cells or the at least one vector comprises an indicator nucleic acid which produces a detectable signal; (d) measuring the amount of the detectable signal produced by the plurality of second cells in order to determine the infectivity of the viral particles; and (e) comparing the amount of signal measured in step (d) for the portions of viral particles obtained from step (a) and step (b),wherein a reduced amount of signal measured for viral particles from step (b) contacted with the antibody as compared viral particles from step (a) maintained in the absence of the antibody indicates that the viruses from the patient comprise viruses which are susceptible to neutralisation by antibody. (a) incubating a plurality of first cells each comprising at least one HIV-1 expression vector which lacks a nucleic acid encoding an HIV-1 viral envelope polypeptide and a nucleic acid encoding an HIV-1 viral envelope polypeptide sequence obtained from a patient, the patient HIV-1 envelope polypeptide sequence comprising extracellular and transmembrane domains of the viral envelope protein, wherein the patient HIV-1 envelope polypeptide sequence does or does not include the intracellular cytoplasmic tail domain or a portion thereof, and wherein the HIV-1 expression vector may comprise nucleic acid sequences encoding a constant predefined cytoplasmic tail domain; such that the plurality of first cells produce viral particles comprising a population of extracellular and transmembrane domains of the viral envelope protein from an HIV-1 virus population from the patient; (b) contacting a portion of the viral particles produced in step (a) with an HIV-1 antibody; (c) contacting the viral particles of steps (a) and (b) separately with a plurality of second cells, wherein the second cells express a cell surface receptor to which the viral particles bind and wherein either the second cells or the at least one vector comprises an indicator nucleic acid which produces a detectable signal; (d) measuring the amount of the detectable signal produced by the plurality of second cells in order to determine the infectivity of the viral particles; and (e) comparing the amount of signal measured in step (d) for the portions of viral particles obtained from step (a) and step (b),wherein a reduced amount of signal measured for viral particles from step (b) contacted with the antibody as compared viral particles from step (a) maintained in the absence of the antibody indicates that the viruses from the patient comprise viruses which are susceptible to neutralisation by antibody.