In order to understand the role of myc family genes (c-myc, N-myc and L-myc) in the development and progression of human testicular cancer, we have analyzed the expression of myc family genes in three different types of primary human testicular cancer (seminoma, embryonal carcinoma and teratocarcinoma) and normal testis using Northern blot analysis. Expression of N-myc gene, which is usually limited in the neoplasms derived from neuroectoderm, was detected in seven out of ten cases of seminomas and two out of two cases of embryonal carcinomas. Gene amplification was not observed in these cases. Expression of N-myc gene was not detected in teratocarcinomas and normal testes. Expression of c-myc gene was observed in seminomas, embryonal carcinomas, teratocarcinomas and normal testes, but specific expression of c-myc gene was not seen in these cancers and normal testes. Expression of L-myc gene was not detected in all cases examined in our studies. Since N-myc gene expression was observed only in undifferentiated testicular neoplasm, such as seminoma and embryonal carcinoma, its expression may be positively related to the development and progression of special types of human testicular cancer.