An attempt to identify the individuality from crossed electro-immunodiffusion (CEID) patterns of serum was made by using coefficient of variation (CV), the ratio of standard deviation to mean of ratios of area in couples of corresponding peaks in the CEID patterns. The values of CV in most cases of sera from identical subject were less than 10 except only one case of 10.6. On the contrary, those in most cases of sera from different subjects were more than 20 except a few cases of 14.9 to 20. No overlap of CV values was observed between the two groups. No significant differences were demonstrated in three groups of sex pairing, i.e., male-male, female-female and female-male. From these results, analysis of CEID patterns by employing CV was found to be available for fairly definite identification as well as differentiation but useless for distinction of sexes in cases of different subjects.