The survival of Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa in human plasma was investigated following their exposure to a sub-inhibitory concentration of ceftriaxone sodium. The aim was to determine the possible effect of human immune agents present in the plasma on microorganisms that may have been previously exposed to sub-inhibitory concentrations of antibiotics. Microbial survival was assessed by viable cell counts conducted at hourly intervals over a period of 5 hours. Subsequently, surviving organisms were used to determine the minimal inhibitory concentration of the antibiotics. The results showed that the rate of replication of P. aeruginosa in plasma was significantly reduced after exposure to a subinhibitory concentration of ceftriaxone sodium. On the contrary, there was an increase in the minimum inhibitory concentration of the antibiotic after K. pneumoniae or E. coli were exposed to the sub-inhibitory antibiotic concentration. This implies that, while some organisms were weakened by sub-lethal doses of ceftriaxone sodium to the extent of susceptibility to human immune agents, others gained resistance to both the antibiotic and the immune system.