Thrombocytopenia is quantitatively correlated with spirochetemia. 129Sv/C57BL6 mice were inoculated intraperitoneally with 10⁶ spirochetes of the indicated B hermsii strain. Spirochetemia, determined as described in “Materials and methods,” peaked on day 2 or 3 after infection and is plotted on the x-axis. Platelet counts were also monitored, and the percentage of decrease from day 0 was calculated and plotted on the y-axis. The correlation coefficient (r) is given. Sets of 2 mice were infected with each strain, and the results from each mouse were similar. For clarity, values from only 1 mouse are shown.