The discovery of additional phenotypic variability in relatively long-established systems of blood groups sometimes comes about rather suddenly after long periods of quiescence. The M-N system of man (Landsteiner and Levine, 1928) is a case in point, For a period of approximately 20 years that system was regarded for all purposes as a two-blood-factor (M and N), two-allele (LM and LN), three-phenotype (M, MN, and N) system, even though variants (Nz) of agglutinogen N were known. Then, beginning with the discovery of blood factor S (Walsh and Montgomery, 1947), the M-N-S system, as it is presently known, has become expanded into a polyphenotype system involving numerous alleles, perhaps well over 30. The F-V system of cattle was, at the time of its inception in 1943, a perfect analogue in infrahuman species of the original M-N system of man. Like M-N, it appeared as a two-blood-factor 5 (normal sheep serum); 6 (normal goat serum); 7 (normal Pronghorn antelope serum); 8 (sheep anticattle); 9 (goat anticattle); 10 (rabbit antihuman A); 11 (cattle antihuman A); 12 (cattle antibison) ; 13 (rabbit antibison) ; 14 (rabbit anticamel); 15 (see Discussion).