A new procedure, plasma fibrinogen chromatography, was utilized, together with other blood coagulation assays, to quantify fibrin formation in 43 children with acute poststreptococcal glomerulonephritis (AGN) from the time of hospitalization until recovery. During the prediuretic phase of AGN, significant evidence for substantial increase in fibrin formation (intravascular coagulation) included gross increase in plasma high MW fibrinogen complexes, the development of either hypo- or hyperfibrinogenemia and gross depression of coagulation factor XIII concentration and of .alpha.-macroglobulin concentration. During the diuretic phase of the disease these abnormalities regressed and evidence of enhanced plasma fibrinolytic activity, documented by an increase in fibrinogen first derivative, was detected. Concomitantly, urinary excretion of fibrin(ogen) degradation products (FDP) underwent substantial increase. With disease recovery, which occurred in all children, urinary FDP excretion ceased and all coagulation findgins normalized.