Clinical findings from 76 patients (median age 67 years) with gram-negative bacteremia were analysed and related to the sensitivity of the blood isolates to the bactericidal activity of normal human serum. 28 strains (37 %) were resistant, an equal number intermediately sensitive and 20 markedly sensitive (26 %). No correlation was found between serum sensitivity and origin of the bacteremia, presence of fever or blood granulocyte count. The frequency of shock in immunocompromised patients with serum-resistant strains was 60 % (6/10); in those with intermediately or markedly sensitive strains it was 44 % (8/18). In the non-immunocompromised patients with resistant strains the frequency of shock was 33 (6/18) versus 10 % (3/30) in those without such strains. Thus the risk of developing shock with gram-negative bacteremia seems to depend on both parasite and host factors, although in this study only the latter were statistically significant. We conclude that serum-sensitive strains can invade the blood stream in spite of the serum bactericidal activity and cause severe disease in some patients.