Studies carried out using either mice or humans have shown that cytotoxic T‐lymphocyte (CTL) responses to many different pathogenic organisms often comprise CTL specific for multiple class I‐restricted pep‐tide epitopes. Differences in the magnitude of epitope‐specific CTL responses appear to arise mainly from differences in the expression level of the corresponding class I/peptide complex on the surface of the antigen‐presenting cell. The size of the CTL response may be United by the frequency and possibly by the affinity of specific CTL precursors in the naive T‐cell pool. Thus, both the efficiency of antigen processing and the composition of the peripheral T‐cell pool impose direct limitations on the extent of a T‐cell response to a given peptide epitope. Studies of CTL hierarchies have resulted in the identification of immunodominant epitopes i.e. peptide epitopes which stimulate the largest number of specific CTL and which are therefore generally believed to offer the best level of protection against the pathogen from which they were derived. It is also thought that CTL responses to non‐dominant epitopes mediate protection against pathogenic challenge. These ideas are considered here with respect to experimental data collected following infection of mice with lymphocytic choriomeningitis virus.