Distribution of marker molecules in discontinuous Percoll gradients. A postnuclear supernatant was divided in two and centrifuged on either a three-step Percoll gradient (a) or a flotation gradient (b) and fractions of 1.5 mL were collected from the bottom of the centrifuge tube. The fractions from the three-step gradient were analyzed for myeloperoxidase (marker for azurophil granules; α; ○), vitamin B₁₂–binding protein (marker for the specific granules; β1; •), gelatinase (marker for specific and gelatinase granules; β1 and β2; □), and total alkaline phosphatase (marker for secretory vesicles and plasma membrane; γ1 and γ2; ▪). The fractions from the flotation gradient were analyzed for latent alkaline phosphatase (marker for the secretory vesicles; γ1; ▿) and nonlatent alkaline phosphatase (marker for the plasma membrane; γ2; ▾). Abscissa, fraction number; ordinate, amount of marker (arbitrary units). Samples for electrophoresis studies were prepared by pooling fractions from the three-step gradient (a) as follows: α, 1-3; β1, 6-8; β2 10-12; and from the flotation gradient (b) as follows: γ1, 10-13; γ2 15-18.