Zfp-37 is a zinc finger protein gene expressed in male germ cells. The cDNA detected two transcripts on Northern blots of testis RNA, with expression first detected at around day 19. To establish the pattern of expression of the protein we have raised antibodies to ZFP-37 and used them on thin sections of testis and on Western blots. On Western blots the antibody detected two proteins exclusively in testis extracts, confirming the previous mRNA expression data. A time-course study revealed that the larger of the two proteins appears at about day 22 but the smaller one is not detected until day 34. Analysis of the expression of these two proteins in purified germ cell preparations revealed that the smaller protein is only detectable in the elongating spermatids or residual bodies. Data from thin sections showed that most, but not all, of the protein recognized by the antibody is in the nucleus, a result further confirmed by Western blotting. These results are discussed in the light of the possible role of this protein in regulating spermatogenesis.