AbstractAn assay based on inhibition of cytopathic effect of human immunodeficiency virus (HIV) strains in Molt 4 cells was developed to quantitate neutralizing antibodies (NA) in sera of HIV‐infected individuals. The assay was specific and gave results comparable to those obtained by the inhibition of immunofluorescence (IFI) and reverse transcriptase (RT) activity. Attempts were made to correlate the presence and the antibody titres with the clinical status of HIV‐infected individuals classified according to Walter Reed staging classification scheme. NA titres correlated inversely with the stage of HIV infection: Compared with acquired immunodeficiency syndrome (AIDS) patients, HIV‐infected subjects at stage WR1 had significantly higher NA titres. Moreover, a decrease in NA titre in relation to clinical deterioration was noted in sequential sera of eight of 11 AIDS patients, retrospectively examined, for NA. The symptomless subjects showed either the same level of NA or a trend towards an increasing antibody titre with time. Different isolates of HIV strains showed a variability in the extent of sensitivity to neutralization by sera obtained from different HIV‐infected indi