Tyrosine phosphorylation of membrane proteins and development of prothrombinase activity in normal erythrocytes and erythrocytes from Scott syndrome incubated with Ca²⁺-ionomycin. Washed cells were incubated with ionomycin in the presence of Ca²⁺ as described in Materials and Methods, and samples were taken at 15 minutes for analysis of tyrosine phosphorylation and at various time intervals for measuring prothrombinase activity as a measure for surface exposed phosphatidylserine. Phosphorylation patterns in red blood cells did not appreciably change on varying the incubation time from 5 to 30 minutes. Upper panel: normal erythrocytes in the absence (lane 1) and presence of Ca²⁺-ionomycin (lane 2) and Scott erythrocytes in the absence (lane 3) and presence of Ca²⁺-ionomycin (lane 4). Lower panel: time course of appearance of prothrombinase activity in normal (▪) and Scott erythrocytes (•).