Serum immunosuppressive acidic protein was compared to lactic dehydrogenase as a marker for testicular cancer in 54 patients with testicular cancer, 62 with benign urological diseases and 203 healthy controls. The mean value of serum immunosuppressive acidic protein in patients with testicular cancer (598 +/- 293 micrograms. per ml.) was statistically higher than that in patients with benign disease (429 +/- 163 micrograms. per ml.) and healthy controls (368 +/- 73 micrograms. per ml.). There were statistically significant differences in serum immunosuppressive acidic protein levels between controls and patients with stage 2 (p less than 0.0001) or stage 3 (p less than 0.001) testicular cancer, and between those with stage 1 and stage 2 (p less than 0.0001) or stage 3 (p less than 0.001) disease, respectively. The usefulness of immunosuppressive acidic protein as a marker for testicular cancer also was compared to that of lactic dehydrogenase. Immunosuppressive acidic protein and lactic dehydrogenase levels were elevated almost equally in patients with stage 2 or 3 disease (range 71 to 92 per cent). However, immunosuppressive acidic protein levels were elevated in only 3 of 25 patients with stage 1 cancer (12 per cent), compared to 11 of 25 (44 per cent) with elevated lactic dehydrogenase levels. Immunosuppressive acidic protein was correlated better with tumor stage. In conclusion, serum immunosuppressive acidic protein determinations may be useful in patients with testicular cancer for staging, monitoring treatment results and predicting recurrence.