To test the hypothesis that there is normally a 14-21 day cycle in blood neutrophil concentration, the blood of 13 normal male volunteers was sampled daily for 42 consecutive days. No evidence for cyclic variation in neutrophil concentration was found. The effect of deleting four samples per week and interpolating values linearly or parabolically was tested by power spectrum analysis, since this type of analysis was used in the studies in which cycling was found. The interpolation of missing values was found to introduce apparent cycling, and the results of the analysis of daily neutrophil counts was no different than that obtained with a series of random numbers.

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