Two neutralizing monoclonal hybridoma antibodies to the surface spike glycoprotein E2 of the JHM strain of murine hepatitis virus were injected into homologous BALB/c mice, and their biological half-lives were determined by sequential titration of plasma samples in a virus-specific enzyme immunoassay. Intravascular half-lives of monomeric immunoglobulins were estimated at 8.0 +/- 1.5 days for antibody 5B19.3, an IgG1, and 12.7 +/- 2.4 days for antibody 4B11.6, an IgG2a. These catabolic rates are statistically different from each other (P less than 0.001) and significantly higher than previously reported values, which were all obtained with radiolabelled polyclonal or myeloma immunoglobulins of unknown specificities. Failure to remove aggregated 4B11.6 antibodies by high-speed centrifugation yielded a statistically significant acceleration of biological turnover (half-life 9.9 +/- 1.6 days; P less than 0.01).