The velocity of precipitin reactions has been measured by following the increase of turbidity of a mixture of antibody and antigen in a spectrophotometer at selected wavelengths from 360 to 720 mμ. For constant amounts of antibody and antigen the measured turbidities are greater at shorter wavelengths. The reaction proceeds fastest in antibody excess and slowest in antigen excess. A lag in the development of turbidity is evident in the equivalence and antigen excess zones. The reaction is speeded up by working in smaller reaction volumes, and by hypotonic concentrations of sodium chloride. Hypertonic concentrations of sodium chloride slow the reaction and cause a lag to become apparent in the antibody excess zone. These effects on the velocity of the reaction are much more pronounced than the effects on the amount of material precipitated. 3,5-Di-iodo-tyrosine slows down the reaction between heavily iodinated bovine albumin and its antibodies.
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