Serial serum C3DP levels of 33 patients at our institution have been followed for up to 10 months. Individuals experiencing periods free of symptoms and signs of proliferating or expanding malignant disease, had C3DP levels which remained below 150 microgram/ml. Patients with active or recurrent disease, while on chemotherapy, had elevated C3DP values (greater than 150 microgram/ml). Serum C3DP values declined abruptly following treatment which resulted in major reduction of tumor mass. Decreases in C3DP levels from values above 150 microgram/ml to values within the normal range (50-150 microgram/ml) were observed during 89% (25/28) of the favorable clinical responses which have been followed with C3DP assays. Increases in C3DP levels from values within the normal range to values above 150 microgram/ml were observed either prior to or coordinate with clinical symptoms of disease recurrence 83% of the time (10/12 cases). These studies suggest that serial C3DP determinations offer an excellent prognostic aid for evaluating the response of malignant tumors during chemotherapeutic management.