This study investigated the influence of bio-social variables - educational status, age and family socio-economic background on teacher union leaders' adherence to democratic principles in Ogun State of Nigeria. The study employed the ex-post-facto research design. Five hypotheses were generated and tested using an instrument titled "union leadership adherence to democratic principles scale" (ULADPS) which comprised 15 items based on the research hypotheses and was structured on a four points Likert scale for data collection. The instrument was administered to a purposely selected population of 200 members of the unions. Data generated were analyzed using Linear Regression Analysis, Analysis of Variance (ANOVA) and Multiple Regression Analysis to test the hypotheses at 0.05 level of significance. The findings show that bio-social variables - educational status, age and family socio-economic background jointly contributed to teacher union leaders' adherence to democratic principles with an F-value of pf 3075.047. The relative predictions of the variables were as follows: educational status (ß = 0.444; P<0.05); age (ß = 0.616; P<0.05) and family socio-economic background (ß = 0.400: P<0.05). It is therefore recommended based on the findings of the study that union leaders should have higher educational qualifications and that matured teachers should lead teacher unions as these will engender willingness to follow democratic principles.