Cross-reactivity studies with Pasteurellaceae from guineapigs revealed 5 serologically distinct groups, comprising Pasteurella multocida, Sp group bacteria, SP-like bacteria, Pasteurella pneumotropica and an actinobacillus-like bacterium. Guineapig Pasteurellaceae differed serologically from mouse-derived P. pneumotropica NCTC 8284. An enzyme-linked immunosorbent assay (ELISA) using SP group antigen, developed to monitor 'natural' infections by SP group Pasteurellaceae in guineapigs, detected significantly more infection than did cultivation, and was found superior to an ELISA performed with P. pneumotropica NCTC 8284.