Fragility of rabbit erythrocytes in agar plates results in gradual release of their NAD and NADP contents into the medium. Due to high NADase and negligible NADPase activity of rabbit red blood cell stroma at neutral pH, the NAD released into the medium is hydrolyzed and NADP remains intact. Thus, rabbit erythrocytes and their lysates support the growth of NAD(P)-requiring Haemophilus by serving as a source of NADP. Stability of sheep erythrocytes in agar plates results in retention of their NAD and NADP contents and consequently in inhibition of growth of NAD(P)-requiring Haemophilus. The highly active NAD- and NADP-splitting enzyme(s) of sheep red blood cell stroma prevent(s) the growth of Haemophilus on sheep blood lysates through inactivation of both NAD and NADP which are released into the medium.