Serum levels of proteins previously shown to be elevated [acute‐phase proteins (APP)—haptoglobin, α₁‐acid glycoprotein, α₁‐antitrypsin] or depressed (α₂HS‐glycoprotein, prealbumin, albumin) in cancer patients were correlated with tumor extent, in vitro lymphocyte reactivity (LR) to phytohemagglutinin (PHA), and quantitative delayed hypersensitivity (DH) to dinitrochlorobenzene (DNCB) in 147 pre‐operative patients with operable solid malignancies either confined to the primary site or with regional spread only. Compared to 58 normal controls, levels of the APP were significantly elevated, α₂HS‐glycoprotein and prealbumin depressed, and albumin levels unchanged in patients with either local or regional tumors. In patients with normal DH to DNCB, the APP were higher and prealbumin was lower than in controls; in patients with impaired DH to DNCB, haptoglobin and α₁‐acid glycoprotein were higher and α₂HS‐glycoprotein and prealbumin lower than in patients with normal DH to DNCB. Albumin levels did not differ from normals in any of the groups. Serum protein levels appeared to be more related to the immune status of the patient than to tumor extent. The levels of the three APP correlated directly with each other but inversely with α₂HS‐glycoprotein and prealbumin; levels of α₂HS‐glycoprotein and prealbumin correlated directly with each other. Levels of haptoglobin and α₁‐acid glycoprotein correlated inversely with LR to PHA; however, levels of α₂HS‐glycoprotein correlated directly with LR to PHA, and uniquely the levels of α₂HS‐glycoprotein and LR to PHA both showed similar changes for each of the four quantitative levels of DH to DNCB measured in the cancer patients. The data show that the proteins studied, except for albumin, correlate inversely (APP) or directly (α₂HS‐glycoprotein and prealbumin) with in vitro and in vivo parameters of cellular immunity. The results provide a rationale for attempts to improve depressed cellular immunity by lowering circulating levels of APP, as is being attempted in ongoing trials using plasmapheresis, and assessing the effect of exogenous α₂HS‐glycoprotein or prealbumin in patients with low levels of these glycoproteins and depressed cellular immunity. The correlations between serum glycoprotein levels and in vitro and in vivo parameters of cellular immunity lend rationale to investigations of the interactions of serum glycoproteins and blood cells having immunologic function that determine the level of cellular immunity expressed in vivo.