Protein adsorption isotherms are often described by the Langmuir equation. Protein adsorption is generally a cooperative process. Although the simple Langmuir equation does not normally include cooperativity, the Hill plot can represent cooperativity, including the degree of cooperativity. Our studies of the adsorption of lysozyme, albumin, transferrin, and lactoferrin onto hydroxyapatite suggest that cooperativity can be described as positive (n > 1.5), partially positive (1 < n < 1.5), partially negative (1 < n < 0.5), and negative (n < 0.5). There are three stages (i, ii, and iii) as protein concentration increases, similar to the cooperativity observed in the interaction between oxygen and hemoglobin. The number of stages is determined by the structural rigidity of proteins and by the lateral interactions between proteins and between protein and adsorbent. Our Hill plots show that stage ii has the lowest degree of cooperativity, stage iii has the highest, and stage i is in between, for most cases. In stage i the low degree of cooperativity is highly consistent with the linear range in the adsorption isotherm when protein solution concentration is low. These results provide useful information for the design and optimization of analytical adsorption chromatography. The factors determining the degree of cooperativity include protein conformation, pI, and solution pH. We now quantitatively understand the cooperative adsorption of these proteins on hydroxyapatite, the degree of cooperativity, and how to control the adsorption process.