Abstract: A method of performing an assay to determine whether a patient has been exposed to or infected by Borrelia burgdorferi is disclosed which comprises collecting serum from the patient; preparing a sample mixture comprising a portion of the patient's serum and an inoculum of viable Borrelia burgdorferi organisms; incubating the sample mixture; determining the number of viable organisms remaining in the sample mixture after incubation; and comparing the number with the quantity of viable organisms remaining in a control. An assay kit is also disclosed which is useful for determining whether a patient has been exposed to or infected by Borrelia burgdorferi. The kit contains reagents necessary to practice the assay method disclosed herein. In its broadest form, the kit comprises an inoculum of viable Borrelia burgdorferi organisms. The kit can also contain an aliquot of normal serum, an aliquot of BSK medium and/or an aliquot of complement. Other reagents, tubes and other materials can also be included in the kits.