The variations in the serum proteins of lobsters were studied by quantitative protein determinations, starch gel and cellulose acetate electrophoresis, and immunodiffusion. A series of bleedings from 15 lobsters provided serum samples for each stage of the molt cycle. Total serum protein was highest before the molt and lowest directly after the molt. The total serum protein averaged 2.5 g/100 ml in the intermolt lobsters and variation among individuals was limited. Variation of serum proteins in an individual during the molt cycle included changes in the electrophoretic and immunodiffusion pattern. Quantitative changes during the cycle indicate a relative shift in the concentrations of various proteins. One component of serum protein was associated only with the development of eggs and another component was present only during premolt and directly after molt.