Porcine NK cells are thought to regulate immune response. Thus, frequency and functional activity of NK cells were examined in generations six to eight (G6, G7, G8) of pigs, selectively bred for high, control, and low immune response. Peripheral blood lymphocytes expressing 5C6 ligand were phenotypically and functionally NK cells. Therefore, using 5C6 as a pan NK phenotype marker and target cell binding and lysis as assays of function, it was found that high and low response pigs had high and low NK cell frequencies respectively at G6 and G8. Functional assays differed by line with control pigs higher or similar to highs which were higher than lows. Vaccination with modified live transmissible gastroenteritis virus did not alter NK cell frequency within or between lines. Vaccination did change functional assay; lows ranked lowest while control and high were similar. Therefore, selection for immune response altered frequency and function of NK cells.