Splice variants of CD44 expressed in a metastasizing cell line derived from a rat pancreatic adenocarcinoma have been shown recently to confer metastatic potential onto non-metastasizing rat pancreatic carcinoma and sarcoma cell lines. Homologues of these variants have also been detected in a variety of human malignancies. Using antibodies raised against a bacterially expressed fusion protein containing variant CD44 sequences, we have explored the expression of variant CD44 glycoproteins on tumors of the female breast. The material examined included normal tissue, hyperplastic lesions, 103 primary invasive mammary carcinomas, 10 in situ carcinomas, 12 local recurrences and 18 lymph node metastases. Using a polyclonal serum directed against several variant CD44 epitopes, normal mammary epithelia as well as ductal hyperplasias were negative for these splice variants, while the variant CD44 epitopes were detectable in all but six of the primary invasive carcinomas. From the reaction with various monoclonal antibodies and polyclonal sera specific for individual epitopes it is obvious that the tumors predominantly express CD44 variants encoded by exons v5 to v7. Interestingly, all investigated lymph node metastases reacted positively with the variant-specific antibodies, in contrast to primary tumors which reacted in 54% to 86% of the cases, depending on the antibody used. Statistical analysis revealed a significant correlation between expression of variant exons v3/v4 and v6 and increased tumor grade (p = 0.001 and p < 0.05, respectively; Fisher's exact test). Exon v6 is carried by the variants which confer metastatic capability in the rat. These results indicate that the expression of the CD44 variants is upregulated in mammary carcinomas and is closely linked to tumor anaplasia.