Pasteurellaceae infection in mice may be monitored by the detection of serum antibody using enzyme-linked immunosorbent assay (ELISA). We re-evaluated our standard antigen panel comprising Pasteurella pneumotropica and a V-factor requiring Haemophilus species (strain H21) by studying their serological relationship with Actinobacillus muris and 'Haemophilus influenzae-murium'. Serologically, A. muris and 'H. influenzae-murium' were found to be unrelated and to differ from P. pneumotropica and Haemophilus strain H21. These four antigens were used for monitoring breeding and experimental mouse colonies for a period of four years. The addition of 'H. influenzae-murium' antigen to the standard panel of antigens significantly increased the proportion of sera and serum panels showing anti-Pasteurellaceae antibody activity, but the addition of A. muris antigen did not.