The myosin heavy chain (MHC) composition of single fibres (n = 2171) was analysed with an electrophoretic technique in biopsy material from m. vastus lateralis of two groups of soccer players before and after a 3-month period of either strength- (n = 8) or non-training (control) (n = 6). Traditional myofibrillar ATPase histochemistry demonstrated a decrease in type IIA fibres with strength-training (35.4 +/- 2.1 vs. 26.7 +/- 2.4% (P < 0.05)). This was not observed in the non-training group (25.7 +/- 4.6 vs. 23.8 +/- 1.7%). One-dimensional electrophoresis on muscle homogenates showed no significant change in the amount of MHC isoforms in either of the two groups. The MHC isoform IIB was undetectable in all but three samples. No changes in the proportions of fibres containing any of the MHC isoforms were observed. Fibres containing only MHC isoform IIB were found in very small numbers (only 11 out of 2171). Before the experimental period, between 6 and 10% histochemical type IIB fibres were found in both groups. This was identical with the proportion of fibres showing co-existence of MHC isoforms IIA and IIB, but in contrast to the very few fibres containing only MHC isoform IIB. This suggests that nearly all histochemical type IIB fibres of the soccer players display co-existence of both MHC isoform IIA and IIB. No major change in the muscle fibre area of the two groups was observed.(ABSTRACT TRUNCATED AT 250 WORDS)