Enzyme-linked immunosorbent assays (ELISAs) and immunoblots were used to measure serum IgG responses in rabbits which were intranasally challenged with Pasteurella multocida. The responses to two serologically distinct isolates (isolate 1, serotype 3:A and isolate 10, serotype 1:D) were compared and then correlated with the ability of the isolates to colonize the nasal passages. Five rabbits were challenged with each isolate (10(5) CFU); nasal washings and sera were collected weekly for 8 weeks. Serum IgG levels were measured by ELISA and immunoblots, using bacterial whole cells and lipopolysaccharides (LPSs) as antigens. The serum IgG response to isolate 1 was evident earlier and was significantly stronger than the response to isolate 10 (P less than 0.025). Immunoblots supported this observation and confirmed that both isolates elicited antibodies which reacted with bacterial protein and LPS antigens, with antibody to protein detectable before antibody to LPS. Results of weekly nasal cultures suggested that the antibody response data could be explained by a difference in the ability of the isolates to colonize the nasal passages: isolate 1 was recovered from four of five rabbits for 8 weeks, whereas isolate 10 was recovered for a maximum of 2 weeks, even when the challenge dose was increased tenfold. The strong response elicited by isolate 1 was therefore probably a result of persistent colonization, whereas the weak response to isolate 10 may have resulted from an inability to persistently colonize the nasal passages. The results of this study demonstrate that isolates of P. multocida elicit antibody responses of differing intensities and vary in their ability to colonize the nasal passages.(ABSTRACT TRUNCATED AT 250 WORDS)