The contributions of four components to the error in the microscopic enumeration of spirochetes (Treponema pallidum) by the method of Morgan and Vryonis were determined. Where N(s) = the number of slides counted, N(r) = the number of recounts of 50 fields per slide, N(p) = the number of pipettes used, and N(e) = the total number of counts of 50 fields made, the coefficient of variation is 2 times the square root of (17.5(2)/N(s) + 18.6(2)/N(r) + 14.8(2)/N(p) + 6.1(2)/N(e)) with 95% confidence.