The erythrocruorin of the leech Haemopis grandis possessed an S20,w of 57 S at neutral pH, its isoelectric point at pH 6.0 and exhibited a slightly sigmoid oxygenation curve with n approximately 2.1 and P50 = 11.2 mm at pH 7.4. A minimum molecular weight of 24000 +/- 1500 per heme group was determined from the iron and heme contents, 0.22 +/- 0.01 and 2.73 +/- 0.14 weight %, respectively. The subunit composition of the erythrocruorin was investigated using gel filtration in sodium dodecyl sulfate and polyacrylamide gel electrophoresis in sodium dodecyl sulfate at neutral pH. Haemopis erythrocruorin dissociated in the presence of sodium dodecyl sulfate into four subunits (1 through 4) possessing molecular weights of about 27000, 23000, 21000 and 13500, respectively. When the erythrocruorin was reduced with mercaptoethanol prior to sodium dodecyl sulfate electrophoresis, three subunits were observed, possessing molecular weights of about 13000 (I), 16500 (II) and 28000 (III). Sodium dodecyl sulfate electrophoresis of the isolated subunits 1 through 4 showed that subunit I was provided by subunits 1 and 4, subunit II was provided by subunit 1 and subunit III was provided by both subunit 2 and subunit 3. Haemopis erythrocruorin thus appeared to consist of at least five different polypeptide chains. It is likely that not all of the constituent polypeptide chains were associated each with a heme group. The shape of the Haemopis erythrocruorin observed by electron microscopy appeared to be consistent with the two-tiered hexagonal array characteristic of annelid erythrocruorins and chlorocruorins.