The preoperatively drawn sera from 84 previously untreated patients who had clinical stage C prostate cancer and underwent staging pelvic lymph node dissections were sent for monoclonal Hybritech analysis to assess the usefulness of prostate specific antigen (PSA) in predicting lymph node status. Of the 84 patients 47 (56%) had positive lymph nodes at surgery. The median PSA value for all patients with nodal metastases was 11.4 ng/.ml., and for those without it was 11.2 ng./ml. Relative to Gleason score, median PSA values were 11.35 for 2-4, 12.2 for 5-7 and 10.9 ng./ml. for 8-10. Within each M.D. Anderson grade median PSA values were 10.15 for grade I, 13.2 for grade II, 12.7 for grade III and 10.5 ng./ml. for grade IV. Simultaneously drawn preoperative frozen serum samples for 28 of these patients were independently analyzed by the Yang radioimmunoassay. Comparing Hybritech and Yang methods revealed strong statistical co-association (correlation coefficient R2 = 97.36, p less than 0.00001) but neither assay was statistically associated with nodal metastasis. Although no PSA level excluded the presence of nodal disease, we suggest that a Hybritech PSA of greater than 30 ng./ml. and a Yang PSA of greater than 50 ng./ml. may serve as a weak adjunctive marker predicting nodal metastasis.