The adult cockroach generates an adaptive humoral immune response exhibiting specificity and immunological memory when immunized with soluble proteins. In contrast, the response induced by bacteria in holometabolous insects is non-specific and short term, generally losing activity after 72 hr. We have found that the roach generates a diphasic response when injected with bacteria, displaying an acute non-specific phase initially, which is then superseded by a second response that is relatively long term and specific. Animals were immunized with either killed Pseudomonas aeruginosa or pyrogen-free Burns-Tracey saline (BTS) and challenged at various times post-immunization (p.i.) with a lethal dose of viable P. aeruginosa. The induction of immunity was measured by monitoring the percentage of survivors after challenge. Immunization with killed P. aeruginosa induced significant (P less than 0.05) protection against challenge as compared with BTS controls. The response was elicited by Day 1 p.i. and did not begin to decline until after Day 14 p.i. The specificity of the response was tested by immunizing animals with either killed Serratia marcescens, Enterobacter cloacae, Streptococcus lactis or Micrococcus lysodeikticus, and then challenging them with live P. aeruginosa. Significant protection against P. aeruginosa challenge was induced by any of the bacteria within the first 3 days p.i. However, by Day 4 the response began to show specificity. Immunization with P. aeruginosa induced significantly more protection than immunization with any one of the other organisms, and only the Gram-negative organisms (P. aeruginosa, E. cloacae and S. marcescens) induced any protection relative to controls.(ABSTRACT TRUNCATED AT 250 WORDS)