A polyvalent 16 serotype Pseudomonas extract vaccine was administered to normal volunteers to ascertain tolerability, immunogenicity and immunisation schedule. Four groups of nine volunteers were immunised with 0.8 X 10(9) bacterial equivalents (BE), 1.2 X 10(9) BE, 1.6 X 10(9) BE or placebo, respectively, on days 0, 14 and 21. A further six volunteers were immunised with 1.6 X 10(9) BE on days 0 and 28. Tolerability was excellent, slight side effects were unrelated to vaccine concentration and decreased with the number of injections. All concentrations of vaccine gave a significantly increased titre 14 days after the first immunisation. Reimmunisation did not increase the titre, which reached a plateau at day 14. Individuals with high pre-immunisation-titres produced very high post-immunisation-titres with a conversion factor of 6.5 whilst those with low pre-immunisation-titres had a higher conversion factor of 13 but produced lower final titres. ELISA titre did not always correlate with biological activity (mouse protection assay) suggesting that effective protective antibodies are only part of the total specific antibody measured by ELISA with the polyvalent whole-cell vaccine antigen.