We investigated whether familial factors influence the plasma content of sex-steroids and sex-hormone-binding globulin (SHBG) in 98 adult males of 66 families. They had no apparent endocrine dysfunction. The 0800–1100 hr plasma levels of testosterone, 5α-dihydrotestosterone (DHT), estradiol-17β (E₂) and estrone (E₁) were measured by radioimmunoassay. The free fractions of E₂ and testosterone were determined by equilibrium dialysis, and the binding capacity of SHBG w as also calculated. The data were analyzed by analysis of variance. We observed that the differences in the plasma content of testosterone (p = .02), SHBG binding capacity (p = .01), and E₂ (p = .03), free E₂ index (p = .05) were all substantially less variable within groups of brothers than among non-brothers. The variability of the plasma concentration of DHT, free testosterone and E₁ was not significantly less within brothers than among non-brothers. The correlation between either plasma testosterone content (r = .14) or SHBG binding capacity (r = .12) and percent of ideal body weight was not significant statistically. Age had no effect on the results. Our data suggest that genetic and/or environmental factors may affect the plasma content of testosterone, E₂ and SHBG binding capacity.