Inbred mice of the CBA and A strains were from weaning malnourished on a low protein diet. During a three-week period their weight increased from about 8 to about 10 g, while mice on normal feeding increased to about 16 g. This malnutrition resulted in a 10-fold increase of the susceptibility to intraperitoneally injected E. coli 06:K2a,2c:H1 bacteria compared to conventionally fed controls. Immunization as well as transfer of fresh immune serum to malnourished or normally-fed mice resulted in an increase in resistance, 6 to 10 fold that of non-immunized controls, while transfer of normal serum did not have a great impact on the resistance. Antibody titers to El coli 06 antigen in mice on poor and normal feeding were similar, as were there non-specific cellular responsiveness measured as lymphoblast transformation by ConA, PHA and LPS. The effect of the malnutrition seemed to be an impairment of the clearance of live E. coli 06 bacteria 1 h after a sub-lethal infection compared with that of normally-fed controls. The noted increased susceptibility to injections in the malnourished mice was rapidly restored by normal feeding for one day.