We studied several blood coagulation parameters and tumor tissue procoagulant activity (PCA) in nude mice bearing human colorectal carcinomas (HCC). In a control group of 51 tumor-free nude mice, platelet number was 1.2 +/- 0.03 x 10(6)/microliters, thrombotest activity 90% +/- 2.6 and fibrinogen 172 +/- 11 mg/dl. The same parameters were studied in nude mice (n = 71) bearing 7 different HCC lines subcutaneously (s.c.). The results did not significantly differ from those in control mice but there was broad variability among groups of mice injected with different HCC lines, ranging from 0.36 to 2.55 x 10(6)/microliters for platelets, from 100 to 28% for thrombotest activity and from 42 to 460 mg/dl for fibrinogen. The results were significantly (p less than 0.05) different from those in the tumor-free group when each group of HCC-bearing animals was analyzed individually. A malignant HCC line that grew in the liver of nude mice (n = 24) significantly (p less than 0.001) reduced thrombotest activity (58% +/- 5.9). The PCA of tissue extracts from tumors grown s.c. in nude mice was assayed. All the HCC xenografts expressed PCA which differed significantly for the various tumor lines (from 25.5 +/- 1.9 to 2.8 +/- 0.6 unit/mg in tumor tissue). Cancer procoagulant (CP), a cysteine proteinase with a direct factor-X-activating effect, was present in different amounts (84.7 +/- 4.3 to 59.5 +/- 9.0%) in the tumors. Our results indicates that the nude mouse is a suitable model for evaluating the hemostatic changes induced by human tumors and may represent a tool for investigating the underlying biochemical mechanisms.