Sucrose gradients have been widely used to study the translational activity of mRNA species in meiotic and haploid spermatogenic cells in mammals. Unfortunately, the results of these studies have been very inconsistent. The purpose of the present study was to obtain accurate and reproducible measurements of the translational activity of a large number of testicular mRNA in sucrose gradients. Extracts of adult testes and cultured seminiferous tubules were sedimented on sucrose gradients, and the distribution of 18 mRNA species was quantified by phosphoimaging. The proportions of various mRNA species sedimenting with polysomes in meiotic and haploid cells (approximately 6-74%) is less than typical of efficiently translated mRNAs (85-90%), demonstrating that the initiation of translation of virtually all mRNA species is at least partially inhibited and that the extent of inhibition is mRNA-specific. Most mRNA species in meiotic and early haploid spermatogenic cells are translated on polysomes in which the ribosome spacing is somewhat wider than in somatic cells, 100-150 verses 80-100 bases. However, the ribosome spacing on protamine mRNAs is unusually close (40-50 bases), and the spacing on poly(A) binding protein mRNA is unusually wide (212-272 bases), thus suggesting that the rate of translational initiation, termination and/or elongation is regulated on translationally active forms of certain mRNA.