BACTERIOTROPIN ESTIMATION (NEUFELD) 215 



If phagocytosis is entirely absent, one should not conclude that bac- 

 teriotropins are not present. Errors in technique are possible: 



(I) Leucocytes may have been injured; this is especially prone to occur 

 if one has worked with heterologous leucocytes; control examinations with 

 homologous leucocytes (from same animal as the serum) should result in 

 phagocytosis. 



(II) The serum concentration may be too high. The use of sensi- 

 tized bacteria will obviate this. Bacteria are first mixed with the serum 

 for a short time, the mixtures centrifugalized, and the serum pipetted off 

 leaving a sediment of sensitized bacteria. 



(III) The time during which the tubes were in the incubator may have 

 been too short or too long. Most micro-organisms require one-half to two 

 hours; pneumococci usually need four hours; cholera vibrios 20 to 30 

 minutes, as they undergo intracellular digestion very readily. 



Neufeld's technique is considered by many simpler than Wright's 

 method. In the determination of normal opsonins, however, concentrated 

 or only slightly diluted sera are employed, thus encountering the difficulties 

 mentioned above (II and III). Homologous leucocytes and sensitized 

 bacteria will remove this interference. 



The editor has found it puzzling at times to decide upon the dilution 

 injwhich bacteriotropins still exist. One may be helped in this decision by 

 the presence or absence of a great number of extracellular organisms. 



