Human glandular kallikrein-I gene (hGK-I) is closely related to the gene of human prostate-specific antigen (PSA) and both genes are expressed in the human prostate. We have studied PSA and hGK-I mRNAs in human prostatic tissue samples from patients with benign prostatic hyperplasia (BPH) or adenocarcinoma (CA), using Northern and slot-blot analysis in order to gain insight into the expression of these highly similar genes. Multiple mRNAs were found to originate from both genes. The major mRNA species of 1.6 kb accounted for 57% to 76% of the total coding capacity for PSA in different tissue specimens, but a variant mRNA species of I .9 kb was also abundant. Most of the BPH samples contained marked amounts of an aberrant 0.9 kb mRNA, and long PSA mRNAs of 6.1 kb, 4.5 kb and 3.1 kb were found in elevated amounts in some of the CA samples. The amount of PSA mRNAs that would produce aberrant PSA proteins if translated into protein varied from 18% to 38% in these tissue samples. The major mRNA species originating from hGK-I was of 1. 6 kb, but other less abundant mRNA species could also be observed. The amount of PSA and hGK-I mRNAs was determined from slot blots hybridized with specific oligonucleotide probes. No significant differences could be found in the PSA gene expression between BPH and CA samples. The total amount of the PSA mRNAs in all the different BPH specimens was fairly similar, but there was a 3-fold difference between the highest and lowest PSA mRNA levels in the CA samples. The hGK-I mRNA levels in the BPH specimens studied demonstrated greater variance than the PSA mRNA levels in the same samples. The correlation between PSA gene and hGK-I expression in the BPH samples was good, suggesting that there are similarities in the regulation of these genes. However, the lack of correlation between the amounts of PSA and hGK-I mRNAs in the CA samples except in sample C I indicates that there are also differences in the gene regulation. The observed 3.7-to 6.5-fold excess of PSA mRNAs as compared with the amount of hGK-I mRNAs present in the same tissue specimen also indicated differences in the cis-or transacting regulatory elements of these genes. 3To whom reprint requests should be addressed.