The present study characterizes the profile of cDNAs from the human P450IIC subfamily in a library from one individual, and it describes three new members of this subfamily (IIC17, IIC18, and IIC19) isolated from two human cDNA libraries. cDNA libraries were constructed from two human livers which differed phenotypically in the hepatic content of P450 HLx (IIC8). The library from the phenotypically low HLx individual was screened by using a cDNA for rat liver P450IIC13 and an oligonucleotide probe for human IIC8. One clone, 245c, was isolated which clearly represents a new member of the human P450IIC subfamily (IIC17). This clone lacked the first 358 nucleotides at the N-terminus but was only 91% homologous in its nucleic acid sequence to IIC9 and 79% homologous to IIC8. Near-full-length clones for IIC9 were also isolated from this library, but no clones for IIC8 were found. Northern blots indicated that the mRNA for IIC8 was low or absent in this individual. A second cDNA library (from a liver phenotypically high in HLx) was then screened. Eighty-three essentially full-length (greater than 1.8 kb) clones belonging to the IIC subfamily were isolated from this library. These include full-length clones for two additional new members of the IIC subfamily. Clones 29c and 6b appear to be allelic variants (IIC18), differing by one nucleotide (one amino acid change) in the coding region. Clone 11a represents a full-length clone for a third new P450 (IIC19).(ABSTRACT TRUNCATED AT 250 WORDS)