Application of the single-dose assay technique to psittacosis.

When suspensions of psittacosis organisms were injected intracerebrally into groups of mice, a nearly linear relationship was observed between the concentration of the organisms injected and the mean time to death of the mice. Thirty-four psittacosis preparations were assayed and, by plotting the relationship between the reciprocal time to death for mice given the 10(-3) dilution of agent against the number of mouse intracerebral median lethal doses (MICLD(50)), a reference curve was established. By use of this reference curve, it was possible to estimate directly the LD(50) value of a psittacosis suspension of unknown concentration from the mean reciprocal time to death of a group of mice injected with a single dilution. By this procedure, the number of mice used was reduced by 62.5%, the titrations were completed in 3 to 5 days as compared with the usual 12 days, three to four times as many assays could be done in 1 day, and no assays had to be repeated. In addition, the precision of the single-dilution assay compared favorably with that of the LD(50) titration.