Optimal blood storage conditions, crossmatching protocol standardization, heterologous transfusion compatibility, and adverse reactions have not been well described in reptile transfusion medicine. This study investigated the effects of blood storage, incubation time, and temperature on crossmatching conducted between clinically normal loggerhead sea turtles (Caretta caretta) [Cc] and green sea turtles (Chelonia mydas) [Cm]. Heparinized venous blood was collected from 17 turtles (n = 12 Cc; n = 5 Cm). Twenty-four homologous (Cc-Cc) donor-recipient pairings and eight heterologous (Cc-Cm) donor-recipient pairings were conducted. Protocols compared different blood storage times of <36 and 120h, incubation times of 30 and 60 min, and incubation at ambient (22.2°C) and refrigerated (7.78°C) temperatures. Major and minor crossmatching hemolysis, macroscopic agglutination, and microscopic erythrocyte agglutination were recorded. No significant differences were found between any protocol tested. Based on major and minor crossmatches, 76.2% (32/42) of homologous crossmatches were compatible and 43.8% (7/16) of heterologous crossmatches were compatible. Two Cc and one Cm that previously received whole blood transfusions did not have different crossmatching outcomes as compared to naive animals (p = 0.4844). This study found a higher crossmatching compatibility between homologous Cc crossmatches than for previously reported homologous Cm crossmatches. Crossmatching using the protocol of 30 min incubation at ambient temperature with blood stored for <36h appeared as an effective method for improving the safety of transfusion medicine in Cc patients. Heterologous compatibility of Cc to Cm is limited, and avoidance of cross-species transfusions is recommended.