This paper describes a relatively simple procedure, the identical pair method, for using existing data to estimate the total number of different antibody V region sequences or isoelectric focusing spectrotypes. Since the calculations required can be performed without a programable computer the method should be useful to all interested biologists, regardless of previous mathematical training. Using this procedure we have determined that the probable repertoire of isoelectric focusing spectrotypes of antistreptococcal group A antibodies from mouse strains A/J and SWR/J are 200 and 300, respectively. The paper also describes how the identical pair method can be used to estimate the number of different V region amino acid sequences. © 1981.