Abstract: A method for expressing proteins which are immunologically reactive with antibodies to lymphadenopathy-associated virus (LAV), now known as Human Immunodeficiency Virus (HIV), is disclosed. The proteins are produced by bacterial host cells transformed with a recombinant plasmid which includes appropriate procaryotic transcriptional and translational signals for expression, followed in reading phase by a DNA sequence comprising a portion of the env region of the LAV genome. This portion codes for a protein which is immunologically reactive with antibodies to LAV, or antibodies to viruses defined to be the same as or equivalent to LAV. The proteins produced by the method disclosed may be used to screen for the presence of antibodies to LAV in a biological fluid, to determine the presence of LAV antigen in a biological fluid, or within a method for producing antibodies to LAV through the immunization of an animal with the protein.