After primary subcutaneous immunization of rabbits with glycolipoprotein from Pseudomonas aeruginosa BI, indirect hemagglutinating and bacterial agglutinating activities appeared in the antiserum 6 days after immunization and reached a peak between 15 and 20 days. Both these in vitro activities paralleled in vivo antipseudomonas-induced leukopenia and mouse passive-protection activities. Further experiments indicated that a functional association exists between the hemagglutinating and passive-protection activities, and that passive protection depends on activity levels in the plasma rather than in the peritoneum. After intraperitoneal injection in mice, in vitro and in vivo activities of antiglycolipoprotein serum declined in the peritoneal cavity as the plasma levels increased. After intravenous injection of the antiglycolipoprotein serum, initially high levels of in vitro and in vivo activity declined at approximately equal rates. Immunoglobulin G (IgG) and immunoglobulin M (IgM) fractions from 15-day antiglycolipoprotein serum were assayed for biological activity. Most of the hemagglutinating and bacterial agglutinating activity and all of the mouse passive-protection activity were found in the IgM fraction. Assay of antiglycolipoprotein serum after 2-mercaptoethanol inactivation of IgM showed that most of the in vitro and all of the passive-protection activities had been destroyed, again locating these activities principally in the IgM fraction of the original antiserum.