The elasticity of substitution between categories of labour is being estimated with new data of the Yugoslav labour market. Model specification for estimation of demand curves of labour undergoes the impact of the Yugoslav socio-economic environment, namely of workers' self-management. The estimated values for the elasticity of substitution between categories of labour imply considerable substitution possibititilies between these categories. The manpower-requirements-approach towards educational planning, most commonly used in Yugoslavia, will therefore have to be discarded by Yugoslav planning authorities. The findings are used to develop the hypothesis that under workers' self-management the degree of substitution between categories of labour is greater than under capitalism.