Because serum exposure precedes interaction of invasive Neisseria gonorrhoeae with neutrophils, neutrophil association with gonococci grown in serum was assessed. Uptake of a nonpiliated serum-resistant strain grown in 20% serum was reduced to 63.5% of control values. Heated serum (20%) yielded similar results. Stimulation of hexose monophosphate shunt activity in neutrophils by heated serum-grown gonococci (a phenomenon reflecting phagocytosis) was 64.5% of control values. Because gonococcal outer membrane (OM) structures mediate interaction with neutrophils, lithium acetate-treated OM preparations of gonococci grown in heated serum were examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Multiple new protein bands and a quantitative decrease in amounts of proteins I, II, and III were noted. However, a decrease in the amounts of these membrane proteins was not observed with alternative membrane extraction techniques. Gonococcal growth in 20% serum dialyzed in 3,500-molecular-weight exclusion tubing allowed normal neutrophil association and OM protein expression.