Physicochemical studies have been carried out on the hemolymph and egg lipoproteins of the rock crab (Cancer antennarius). Analytical ultracentrifugal analyses of vitellogenic female HDL3 revealed the presence of two types of lipoproteins. The first with a sedimentation rate of 5.35 S was comparable to lipoproteins in male and non-vitellogenic female hemolymph. The second with a sedimentation rate of 10.74 S was comparable to the major lipoprotein of egg yolk. A similar comparison could be made following electrophoretic analyses in native polyacrylamide gels. Electrophoresis in SDS-polyacrylamide gels revealed three major apolipoproteins common to egg and vitellogenic HDL3. A fourth apolipoprotein was found in both male and female HDL3. In contrast to mammalian HDL, none of these crustacean apolipoproteins had a molecular weight less than 82 000. One of these apolipoproteins appears to be comparable physicochemically to the enteric form of apolipoprotein B in mammals.